[koonsolo]

You can see it as the "signature" of the creator. Everybody owns the photograph, but only 1 person owns the signed photograph.

[Winsaucerer]

It seems to me more analogous to a certificate, since there's nothing like a signature embedded in the image itself that only one person has. Anyone can have the thing itself (an identical digital copy), but only one person has the certificate provided by the original creator.