The public block chain would show the chain of custody/ownership, so the photo of a photo would show the the final crypto signature does not belong to the claim

The public block chain would show the chain of custody/ownership, so the photo of a photo would show the the final crypto signature does not belong to the claimed owner. You are correct that I as a viewer can't just rely on a crypto-signature like a watermark, I'd have to verify the chain of custody, but if I wanted to do that, it is available to do so.