memory overflow

[LazyCuteLion]

when switch source,seems the old value does not dispose:the memory has not decreased. (wait some minutes also not)
the code is very simple and nothing special:

private void Button_Click(object sender, RoutedEventArgs e)
{
    var dialog = new OpenFileDialog();
    if (dialog.ShowDialog() == true)
    {
        var source = new BitmapImage(new Uri(dialog.FileName));
        ImageBehavior.SetRepeatBehavior(img, System.Windows.Media.Animation.RepeatBehavior.Forever);
        ImageBehavior.SetAutoStart(img, true);
        ImageBehavior.SetAnimatedSource(img, source);
    }
}