Yeah, long term it will be near impossible to tell by inspecting the image.
We will end up with a system of identifying and trusting the image creator to be truthful, and tracking which/if modifications have been done to the picture.
We have cryptography tools that can be used for most of that, but how it will work as a system for all the weird problems people have (like dying, grifting, stealing, selling access, lying, infringing) is not something we can just throw some tech on and assume it solved.