Ah. I’ve used too much to OS that detects file type not solely by file extension
You’ll need to ask NWC employers for that.
Creatures shared between towns won’t be able to have town-specific background - same creature must have same parameters (including icons) no matter where it was recruited.
Something like “composite image” may be an interesting idea but it must be implemented on more basic level and not be something specific to creature icons. Probably as post-1.0 feature (or when it will be possible to implement this easily)