Do a Google search on "watermarking" and understand that ANYTHING posted to the web can be copied and printed freely without limitation by anyone. The only thing you can do is limit the commercial value of posted images by limiting the resolution and/or watermarking the images.

Here is a mini-tutorial explaining one way to do it:

http://www.outbackphoto.com/workflow/wf_54/essay.html

Cheers,
Jack

How do I sign digital photos posted to web site. How do I make it so people can't just take copies of photos I post on a web site?  :p

http://www.photofocus.com/shownews.php?nid=24