Optimize png assets

[ace-dent]

I'd like to run my tool (pngslim) on the png assets, to minimize overall bytes.
This is lossless optimization- but can come in 2 flavours:

1. RGB(A) colour mode – quaranteed to work on everything, even your year 2000 devices.
2. Various colour modes (including indexed with a palette) – always part of the PNG spec', smallest possible files, but ancient devices that were not compliant may not support it.

Happy to provide a PR.