You’re quite correct your thinking it seems unrealistic to actually detect AI generated imagery after the fact so the only fill solution would be a trusted chain of custody - it wouldn’t necessarily need a centralized authority but it would require some highly trusted issuers of trust and, unfortunately, trust in media is currently at an all time low and those companies are in the best position to serve as those issuers.

This is a very complicated problem and we need a social (rather than a technical) solution.