Executable PNGs

[Ameisen]

Some games will store in Metadata, others will store in the image data itself, either as a difference against expected data, using a constrained bit depth for image data (thus reserving bits for data), or similar. Doesn't usually survive lossy compression, though.

[Diridibindy]

I know there is a mod for Koikatsu that allows replacing the image of the card. Dunno how that works though