MindBender gave you the basic legal agreement embodied in US law. However, as he points out, proof is a murky issue. The first and cheapest step that one can take to begin documenting ownership of an image is to put a copy with a description and claim of ownership in an envelope and send it to yourself via registered mail. When it's delivered you file it unopened. Should the case go to court you have documented evidence of creation and ownership which is formally dated.
The next level of protection which takes a bit more effort is to obtain and file with the US Copyright Office
http://www.loc.gov/copyright/ which will file a copy of the work and provide you with a dated certificate of ownership of the copyright. This doesn't protect you from having an image stolen. What it does do is make it more likely that a lawyer will accept your case if litigation becomes an issue.
Lawyers! You've gotta love 'em!