You can either edit Images, XML and TXT files or add entirely new ones.

EDITTING:
It doesn't matter if you want to edit something in assets/shared/images/ or assets/preload/images/,
you will have to put the editted files in mods/images/, it will be handled automatically by the engine.

You can also add your own characters json files!
Create a "characters" folder here and put your json files there.