diff --git a/src/tests/test_image_utils.py b/src/tests/test_image_utils.py
new file mode 100644
index 0000000..8bf3833
--- /dev/null
+++ b/src/tests/test_image_utils.py
@@ -0,0 +1,399 @@
+import base64
+import io
+import socket
+from unittest.mock import AsyncMock, MagicMock, patch
+
+import httpx
+from PIL import Image, UnidentifiedImageError
+import pytest
+
+from app.image_utils import (
+    MAX_FILE_SIZE,
+    MAX_IMAGE_PIXELS,
+    MAX_REDIRECTS,
+    SafeNetworkBackend,
+    _decode_and_convert_image,
+    is_safe_url_async,
+    load_image_from_source,
+    resolve_safe_url_async,
+)
+
+
+def generate_sample_image_bytes(
+    format="PNG", size=(50, 50), color=(255, 0, 0)
+) -> bytes:
+    """Helper to generate valid image bytes."""
+    img = Image.new("RGB", size, color)
+    buf = io.BytesIO()
+    img.save(buf, format=format)
+    return buf.getvalue()
+
+
+async def _async_iter(items):
+    """Async generator helper for mocking aiter_bytes."""
+    for item in items:
+        yield item
+
+
+# ============================================================================
+# 1. Tests for resolve_safe_url_async and is_safe_url_async
+# ============================================================================
+
+
+@pytest.mark.anyio
+async def test_resolve_safe_url_async_valid_public_ip():
+    """Test public URL resolution returns safe=True and resolved IP."""
+    addr_info = [(socket.AF_INET, socket.SOCK_STREAM, 6, "", ("93.184.216.34", 0))]
+    with patch("socket.getaddrinfo", return_value=addr_info):
+        is_safe, ip = await resolve_safe_url_async("https://example.com/image.png")
+        assert is_safe is True
+        assert ip == "93.184.216.34"
+
+        safe_bool = await is_safe_url_async("https://example.com/image.png")
+        assert safe_bool is True
+
+
+@pytest.mark.anyio
+@pytest.mark.parametrize(
+    "private_ip",
+    [
+        "127.0.0.1",
+        "10.0.0.1",
+        "172.16.0.1",
+        "192.168.1.1",
+        "169.254.169.254",
+        "::1",
+    ],
+)
+async def test_resolve_safe_url_async_private_ips(private_ip):
+    """Test private, loopback, and link-local IP addresses are blocked."""
+    family = socket.AF_INET6 if ":" in private_ip else socket.AF_INET
+    addr_info = [(family, socket.SOCK_STREAM, 6, "", (private_ip, 0))]
+    with patch("socket.getaddrinfo", return_value=addr_info):
+        is_safe, ip = await resolve_safe_url_async("http://internal-host/image.png")
+        assert is_safe is False
+        assert ip is None
+
+        safe_bool = await is_safe_url_async("http://internal-host/image.png")
+        assert safe_bool is False
+
+
+@pytest.mark.anyio
+@pytest.mark.parametrize(
+    "invalid_url",
+    [
+        "ftp://example.com/image.png",
+        "file:///etc/passwd",
+        "http://",
+        "not_a_url",
+        "",
+    ],
+)
+async def test_resolve_safe_url_async_invalid_urls(invalid_url):
+    """Test invalid schemes, empty hosts, and malformed URLs are blocked."""
+    is_safe, ip = await resolve_safe_url_async(invalid_url)
+    assert is_safe is False
+    assert ip is None
+
+
+@pytest.mark.anyio
+@pytest.mark.parametrize(
+    "exception_cls",
+    [
+        socket.gaierror("DNS resolution failed"),
+        ValueError("Invalid address format"),
+        TypeError("Invalid type"),
+    ],
+)
+async def test_resolve_safe_url_async_exceptions(exception_cls):
+    """Test socket / resolution exceptions are safely caught and return (False, None)."""
+    with patch("socket.getaddrinfo", side_effect=exception_cls):
+        is_safe, ip = await resolve_safe_url_async("https://invalid-domain.example")
+        assert is_safe is False
+        assert ip is None
+
+
+# ============================================================================
+# 2. Tests for SafeNetworkBackend and _decode_and_convert_image
+# ============================================================================
+
+
+@pytest.mark.anyio
+async def test_safe_network_backend():
+    """Test SafeNetworkBackend redirects connect_tcp calls to the designated safe IP."""
+    mock_backend = AsyncMock()
+    safe_backend = SafeNetworkBackend(mock_backend, safe_ip="93.184.216.34")
+
+    # connect_tcp
+    await safe_backend.connect_tcp("example.com", 443, timeout=5.0)
+    mock_backend.connect_tcp.assert_called_once_with(
+        "93.184.216.34",
+        443,
+        timeout=5.0,
+        local_address=None,
+        socket_options=None,
+    )
+
+    # connect_unix_socket
+    await safe_backend.connect_unix_socket("/tmp/socket.sock", timeout=2.0)
+    mock_backend.connect_unix_socket.assert_called_once_with(
+        "/tmp/socket.sock", timeout=2.0, socket_options=None
+    )
+
+    # sleep
+    await safe_backend.sleep(0.5)
+    mock_backend.sleep.assert_called_once_with(0.5)
+
+
+def test_decode_and_convert_image_valid():
+    """Test _decode_and_convert_image decodes valid image bytes to RGB PIL Image."""
+    png_bytes = generate_sample_image_bytes(
+        format="PNG", size=(20, 20), color=(0, 255, 0)
+    )
+    image = _decode_and_convert_image(png_bytes)
+    assert isinstance(image, Image.Image)
+    assert image.mode == "RGB"
+    assert image.size == (20, 20)
+
+
+def test_decode_and_convert_image_corrupted():
+    """Test _decode_and_convert_image raises exception on corrupted bytes."""
+    corrupted_bytes = b"not an image"
+    with pytest.raises((UnidentifiedImageError, OSError)):
+        _decode_and_convert_image(corrupted_bytes)
+
+
+# ============================================================================
+# 3. Tests for load_image_from_source (Base64)
+# ============================================================================
+
+
+@pytest.mark.anyio
+async def test_load_image_from_source_base64_success():
+    """Test loading valid Base64 image data URL."""
+    png_bytes = generate_sample_image_bytes(format="PNG", size=(30, 30))
+    b64_str = base64.b64encode(png_bytes).decode("utf-8")
+    data_url = f"data:image/png;base64,{b64_str}"
+
+    async with httpx.AsyncClient() as client:
+        img = await load_image_from_source(data_url, client)
+        assert isinstance(img, Image.Image)
+        assert img.mode == "RGB"
+        assert img.size == (30, 30)
+
+
+@pytest.mark.anyio
+async def test_load_image_from_source_base64_exceeds_size():
+    """Test Base64 image payload exceeding MAX_FILE_SIZE raises ValueError."""
+    large_data = b"X" * (MAX_FILE_SIZE + 100)
+    b64_str = base64.b64encode(large_data).decode("utf-8")
+    data_url = f"data:image/png;base64,{b64_str}"
+
+    async with httpx.AsyncClient() as client:
+        with pytest.raises(
+            ValueError, match="画像サイズが上限\\(15MB\\)を超えています"
+        ):
+            await load_image_from_source(data_url, client)
+
+
+@pytest.mark.anyio
+async def test_load_image_from_source_base64_invalid():
+    """Test invalid Base64 string raises ValueError."""
+    data_url = "data:image/png;base64,invalid_base64_content!!!"
+
+    async with httpx.AsyncClient() as client:
+        with pytest.raises(ValueError, match="Base64画像のデコードに失敗しました"):
+            await load_image_from_source(data_url, client)
+
+
+# ============================================================================
+# 4. Tests for load_image_from_source (URL Download & Redirects)
+# ============================================================================
+
+
+@pytest.mark.anyio
+async def test_load_image_from_source_url_ssrf_blocked():
+    """Test URL resolving to private IP is rejected by SSRF check."""
+    with patch("app.image_utils.resolve_safe_url_async", return_value=(False, None)):
+        async with httpx.AsyncClient() as client:
+            with pytest.raises(
+                ValueError, match="セキュリティ上の理由で拒否されたURLです"
+            ):
+                await load_image_from_source("http://127.0.0.1/image.png", client)
+
+
+@pytest.mark.anyio
+async def test_load_image_from_source_url_success_mock_client():
+    """Test URL stream download happy path with custom/mocked client."""
+    img_bytes = generate_sample_image_bytes(format="JPEG", size=(40, 40))
+
+    mock_resp = MagicMock()
+    mock_resp.is_redirect = False
+    mock_resp.raise_for_status = MagicMock()
+    mock_resp.aiter_bytes = MagicMock(return_value=_async_iter([img_bytes]))
+
+    mock_stream_ctx = AsyncMock()
+    mock_stream_ctx.__aenter__.return_value = mock_resp
+
+    mock_client = MagicMock()
+    mock_client.stream.return_value = mock_stream_ctx
+
+    with patch(
+        "app.image_utils.resolve_safe_url_async",
+        return_value=(True, "93.184.216.34"),
+    ):
+        img = await load_image_from_source("https://example.com/image.jpg", mock_client)
+        assert isinstance(img, Image.Image)
+        assert img.size == (40, 40)
+
+
+@pytest.mark.anyio
+async def test_load_image_from_source_url_success_real_async_client():
+    """Test URL stream download happy path when passing standard httpx.AsyncClient."""
+    img_bytes = generate_sample_image_bytes(format="PNG", size=(25, 25))
+
+    mock_resp = MagicMock()
+    mock_resp.is_redirect = False
+    mock_resp.raise_for_status = MagicMock()
+    mock_resp.aiter_bytes = MagicMock(
+        return_value=_async_iter([img_bytes[:100], img_bytes[100:]])
+    )
+
+    mock_stream_ctx = AsyncMock()
+    mock_stream_ctx.__aenter__.return_value = mock_resp
+
+    with patch(
+        "app.image_utils.resolve_safe_url_async",
+        return_value=(True, "93.184.216.34"),
+    ):
+        with patch("httpx.AsyncClient.stream", return_value=mock_stream_ctx):
+            real_client = httpx.AsyncClient()
+            img = await load_image_from_source(
+                "https://example.com/test.png", real_client
+            )
+            assert isinstance(img, Image.Image)
+            assert img.size == (25, 25)
+
+
+@pytest.mark.anyio
+async def test_load_image_from_source_url_exceeds_max_file_size():
+    """Test streamed response exceeding MAX_FILE_SIZE raises ValueError."""
+    chunk1 = b"A" * 10
+    chunk2 = b"B" * 10
+
+    mock_resp = MagicMock()
+    mock_resp.is_redirect = False
+    mock_resp.raise_for_status = MagicMock()
+    mock_resp.aiter_bytes = MagicMock(return_value=_async_iter([chunk1, chunk2]))
+
+    mock_stream_ctx = AsyncMock()
+    mock_stream_ctx.__aenter__.return_value = mock_resp
+
+    mock_client = MagicMock()
+    mock_client.stream.return_value = mock_stream_ctx
+
+    with (
+        patch(
+            "app.image_utils.resolve_safe_url_async",
+            return_value=(True, "93.184.216.34"),
+        ),
+        patch("app.image_utils.MAX_FILE_SIZE", 15),
+    ):
+        with pytest.raises(
+            ValueError, match="画像サイズが上限\\(15MB\\)を超えています"
+        ):
+            await load_image_from_source("https://example.com/large.png", mock_client)
+
+
+@pytest.mark.anyio
+async def test_load_image_from_source_url_redirect_success():
+    """Test following a valid safe redirect location."""
+    img_bytes = generate_sample_image_bytes(format="PNG", size=(15, 15))
+
+    # First response: 302 redirect
+    mock_redirect_resp = MagicMock()
+    mock_redirect_resp.is_redirect = True
+    mock_redirect_resp.headers = {"Location": "https://example.com/redirected.png"}
+    mock_redirect_resp.url = httpx.URL("https://example.com/initial.png")
+
+    mock_redirect_ctx = AsyncMock()
+    mock_redirect_ctx.__aenter__.return_value = mock_redirect_resp
+
+    # Second response: 200 OK
+    mock_ok_resp = MagicMock()
+    mock_ok_resp.is_redirect = False
+    mock_ok_resp.raise_for_status = MagicMock()
+    mock_ok_resp.aiter_bytes = MagicMock(return_value=_async_iter([img_bytes]))
+
+    mock_ok_ctx = AsyncMock()
+    mock_ok_ctx.__aenter__.return_value = mock_ok_resp
+
+    mock_client = MagicMock()
+    mock_client.stream.side_effect = [mock_redirect_ctx, mock_ok_ctx]
+
+    with patch(
+        "app.image_utils.resolve_safe_url_async",
+        return_value=(True, "93.184.216.34"),
+    ):
+        img = await load_image_from_source(
+            "https://example.com/initial.png", mock_client
+        )
+        assert isinstance(img, Image.Image)
+        assert img.size == (15, 15)
+        assert mock_client.stream.call_count == 2
+
+
+@pytest.mark.anyio
+async def test_load_image_from_source_url_redirect_missing_location():
+    """Test redirect response without Location header raises ValueError."""
+    mock_redirect_resp = MagicMock()
+    mock_redirect_resp.is_redirect = True
+    mock_redirect_resp.headers = {}
+
+    mock_redirect_ctx = AsyncMock()
+    mock_redirect_ctx.__aenter__.return_value = mock_redirect_resp
+
+    mock_client = MagicMock()
+    mock_client.stream.return_value = mock_redirect_ctx
+
+    with patch(
+        "app.image_utils.resolve_safe_url_async",
+        return_value=(True, "93.184.216.34"),
+    ):
+        with pytest.raises(
+            ValueError, match="リダイレクト先Locationヘッダーが指定されていません"
+        ):
+            await load_image_from_source(
+                "https://example.com/bad-redirect.png", mock_client
+            )
+
+
+@pytest.mark.anyio
+async def test_load_image_from_source_url_max_redirects_exceeded():
+    """Test exceeding MAX_REDIRECTS raises ValueError."""
+    mock_redirect_resp = MagicMock()
+    mock_redirect_resp.is_redirect = True
+    mock_redirect_resp.headers = {"Location": "https://example.com/loop.png"}
+    mock_redirect_resp.url = httpx.URL("https://example.com/loop.png")
+
+    mock_redirect_ctx = AsyncMock()
+    mock_redirect_ctx.__aenter__.return_value = mock_redirect_resp
+
+    mock_client = MagicMock()
+    mock_client.stream.return_value = mock_redirect_ctx
+
+    with patch(
+        "app.image_utils.resolve_safe_url_async",
+        return_value=(True, "93.184.216.34"),
+    ):
+        with pytest.raises(ValueError, match="リダイレクト回数が上限を超えました"):
+            await load_image_from_source("https://example.com/loop.png", mock_client)
+
+    # MAX_REDIRECTS + 1 = 4 calls total
+    assert mock_client.stream.call_count == MAX_REDIRECTS + 1
+
+
+def test_max_image_pixels_config():
+    """Verify decompression bomb guard setting is set to 20 megapixels."""
+    assert MAX_IMAGE_PIXELS == 20_000_000
+    assert Image.MAX_IMAGE_PIXELS == 20_000_000