Humans will need to use digital signatures eventually. Chains of verifiable claims from real humans would be used. Still doesn't prove anything by itself, but it saves a ton of effort. That, plus verifiable timestamping.

And as AI gets better and better, original source material is going to become harder to access and harder to verify unless you can lay your hands on a real piece of paper that says it’s so.

One of the bright lines between Existing Art and AI Art, particularly when it comes to historical photos and other images, is that there typically isn't a physical copy of the original. You're not going to walk into the Louvre and have this problem.

This brings up another complication in the art world, which is ownership/right-to-reproduce said image. Blindly crawling the internet and vacuuming up whatever you find, then labeling it as you find it, has been a great way for search engines to become functional repositories of intellectual property without being exposed to the costs associated with reprinting and reproducing. But all of this is happening in a kind-of digital gray marketplace. If you want the official copy of a particular artwork to host for your audience, that's likely going to come with financial and legal strings attached, making its inclusion in a search result more complicated.

Since Google leadership doesn't want to petition every single original art owner and private exhibition for the rights to use their workers in its search engine, they're going to prefer to blindly collect shitty knock-offs and let the end-users figure this shit out (after all, you're not paying them for these results and they're not going to fork out money to someone else, so fuck you both). Then, maybe if the outcry is great enough, they can charge you as a premium service to get more authentic results. Or they can charge some third party to promote their print-copies and drive traffic.

But there's no profit motive for artistic historical accuracy. So this work isn't going to get done.