By placing the copyright stamp across the actual image, you lessen the chances someone will want to steal it off the net.
If a person needed a shot of say "Russell Falls, Tasmania" and they search for it, they will choose to take a copy of the non copyright stamped one over the one that is. Its not an ultimate solution, but it can go part way to protecting your photos.
How to do it, read my post (third one down) in this thread: http://www.ausphotography.net.au/for...ead.php?t=6132