Forum Moderators: not2easy
For some insight about digital watermarks, see the post by WebmasterWorld member "Leosghost" in the following thread:
http://www.webmasterworld.com/forum44/780-2-15.htm [webmasterworld.com]
There are also more threads about digital watermarks [google.com] on WebmasterWorld. Most deal with how to protect one's own work, but some will provide information on potential methods to verify if an image has been protected.
When in doubt, ask for proof. Don't believe someone has rights just because they say so. If someone dropped off a car on my doorstep and told me it was mine to have, I'd want some verifiable proof the car was theirs to give (not stolen or borrowed) before I went cruising the town in it or parked it in my driveway.