If you’re considering watermarking your images, read Kyle’s article on what to consider. At About.com, he discusses types of watermarking, and whether to even do it or not. He has written a lightly-detailed, easy to read article that can help you decide what is best for your pictures. Watermarking can help prevent theft, but depending on how you use your pics, it can also hurt your image(s).
How To Watermark Your Photos by Kyle Shurman
Rebecca guides you through watermarking with Photoshop.
Photoshop Tutorial: Adding A Watermark To Your Images by Rebeccca Baer