This is a problem of provenance (as it's known in the art world) and being certain of the provanence is a difficult thing to do - it's like converting a cowboy coded C++ project to consistently using const... you need to dig deep into every corner and prefer dependencies that obey proper const usage. Doing that as an individual content creator would be extremely daunting - but this isn't about individuals. If Getty has a policy against AI and guarantees no AI generation on their platform while Shutterstock doesn't[1] then creators may end up preferring Getty so that they can label their otherwise AI free content as such on Youtube - maybe it gets incorporated into the algorithm and gets them more views - maybe it's just a moral thing... if there's market pressure then the down-the-chain people will start getting stricter and, especially if one of those intermediary stock providers violates an agreement and gets hit with a lawsuit, then we might see a more concerted movement to crack down on AI generation.

At the end of the day it's going to be drenched in contracts and obscure proofs of trust - i.e. some signing cert you can attach to an image if it was generated on an entirely controlled environment that prohibits known AI generation techniques - that technical side is going to be an arms race and I don't know if we can win it (which may just result in small creators being bullied out of the market)... but above the technical level I think we've already got all the tools we need.

1. These two examples are entirely fabricated