So far, only @sam’s image://sha1.png sounds good to me. And I don’t feel any strong love for that. Why do you care how clean or ugly a filename a user doesn’t have to type in themselves is?
All the other ideas seem like more complexity for little gain. The whole idea of using a hash is that identical uploads can easily be optimized for disk space. Changing to some other ID doesn’t achieve that. You could store and save the SHA1 in the background and give out the same GUID for it, but there’s that complexity.
(And using MD5 is bad since there’s a non-zero chance someone will want to post deliberate collisions. But as easily forced SHA1 collisions will probably be upon us soon, too, maybe the complexity would be good?)