If you want to help protect your images from being stolen and used without your knowledge, permission, and/or financial compensation, then I suggest that you consider registering the images with the US Copyright Office (
http://www.copyright.gov/eco/). The fee for on-line registration is $35, and you can register multiple images under one filing. Carolyn E. Wright (her web site:
www.photoattorney.com) has put together a nice tutorial on how to go through the on-line filing with the US Copyright Office. You can view the tutorial here:
www.naturescapes.net/docs/index.php/articles/341.
I have gone through this process a couple of times and it is fairly simple to do. Remember, you own the copyright to the image as soon as you press the shutter release button on your camera (unless you are a work-for hire contract - like a newspaper photojournalist). Registering your images enables you to seek damages from users infringing on your image copyrights. Registering your image copyright does not prevent the image from being stolen and used, but it does enable you additional legal recourse.
Disclaimer: I am not a lawyer, and the above information is not intended to be legal advice (just information). When in doubt about copyright infringement seek the appropriate legal advice.