As far as I am aware, artwork is automatically copyrighted [I am from U.K], if you find your work somewhere, you can sue , without having signed or dated anything.
If you go to an art gallery you will see none of the paintings have the copyright sign, because it is not needed.The artist is well protected [ sign and date the backs of all my work]...I am not sure about computer generated art, in the sense of being able to prove its your creation, with a painting or drawing this is easy to do.....you would have the original.
If you find your work somewhere and can prove its your work....then the person using it would have to show a contract , from you, showing that you gave permission.[ note read small print someimes when submitting work, you could sign it away without knowing it
]
I think this automatic copyright lasts about 50 years after death [not sure
], after that it becomes un-copyrighted...[books and classical music are a good example]
[stuff on the net I tend to keep to a low rez, or when doing high rez I put the 'c'.
http://mysite.freeserve.com/418_TAROT/
I think many websites you get automatic copyright?