Possibly, but should be easy enough to exclude. For example Dall-e has the pixels watermark (bottom-right), and I assume there's a possibility of an indicator

Possibly, but should be easy enough to exclude. For example Dall-e has the pixels watermark (bottom-right), and I assume there's a possibility of an indicator that may by hidden in the data itself. One could also exclude common meme formats and their derivatives. Then there's the option of mapping to produced content via hashing a la Shazam, or have a discriminator component etc. But you're right, it's not trivial. I just don't think it's too big of a deal.