I don't understand how adding your username is a good indication of proof. Admittingly, sig related stuff isn't my forte, but what's stopping a person from just cropping out someone's name and typing their own in its place? Theoretically, couldn't I go to another forum, take someone's sig, photoshop it a bit, and claim it as my own? It doesn't strike me as a reliable safeguard against stealing.
Err, I should add that I'm not accusing anyone or that I believe anyone here would cheat, it's just something I was curious about and wanted to bring up.
Suggest a more suitable approach and it'll be noted. So far I haven't thought of anything.
And no, I don't want to have 100s of PSDs to look through, and it'll overcomplicate the system.