Awesome, glad you want to help out! The images are defined by a unit's armor, so you need to find which armor that unit has, then look for which sprites in extraSprites it is. The image files are found in the mod's Resources folder - look in the extraSprites definitions for which subfolder. The battlescape sprites are difficult to look at from source file though - I think IvanDogovich knows a good way of getting those images on the wiki.