What's the threat vector you're trying to mitigate here? If you're wondering whether a movie that claims to be produced by Disney really was, if it's in theaters or on Disney+, then you can trust it was actually made by Disney or at least licensed to them. As long as the Washington Post still employs its own photographers and doesn't accept imagery submission from the general public, you should be able to trust a photo published in the Washington Post is at least of something real, unless you just don't trust the Post itself. If you're thinking a YouTube channel or something, unless the channel got hacked, seemingly anything published there was really published by the channel owner. Maybe they're showing you something made by AI that isn't real, but as the owner of their own signing key, nothing would prevent them from signing an AI-generated image.

If you're talking someone on Twitter or Facebook is putting a photo in your feed claiming a human photographed BLM throwing bricks through a window, don't trust shit being posted on Facebook or Twitter no matter what, probably, but even there, unless the profile was hacked, you either trust the person who owns it or you don't. Nothing would prevent them from signing a forgery of reality that they legitimately forged themselves. Even with device-level keys, what are you trying to prove? You can pay actors to throw bricks through windows.

I guess the concern is this doesn't scale as well as asking Midjourney to do it, but I wonder to what extent that is even true. With 8 billion people on the planet and counting and a whole lot of them doing this shit, given the limited input bandwidth of human sense organs, there is seemingly some maximum saturation of bullshit a person can be exposed to that a lot of people have already hit, and having the Internet host even more of it doesn't mean they'll grow bigger eyes and a faster brain that can actually ingest more bullshit than it already does.