Actually would it be possible for them to make gun covers (Like the old cell phone covers) where you can snap on or otherwise attach customized images? Do they already have those?
Firearms get hot when fired, so putting anything like that on the metal parts would be probably not a good idea. However, on the buttstock you could probably put whatever you want, with the caveat that it would probably make it harder to fire if it added any bulk or made the weapon sit funny.