That fixes the problem of content being manipulated and then the original being discounted as fake when challenged. It doesn't do a whole lot for something ent

That fixes the problem of content being manipulated and then the original being discounted as fake when challenged. It doesn't do a whole lot for something entirely fictional, unless it becomes so ubiquitous that anything unsigned is assumed to be fake rather than just made on a "normal" device. And even if you did manage to sign every photo, who's managing those keys? It's the difference between TLS telling you what you see is what the server sent and trusting the server to send the truth in the first place.