Sorry to jump into this thread so late. No one has mentioned the infamous right-click-blocker Javascript, which of course can't prevent the user from going into his/her browser cache to retrieve the image, but then at least it takes a little more effort to do so.
I recently met a photog who told me that he gives all his images simple file names, and then he does periodic searches for them online. He says that people won't bother to change image names that are simple, and he has caught people using that method. He once found an image of his being used by a bank, and he took legal action against them. - This message is from the Pentax-Discuss Mail List. To unsubscribe, go to http://www.pdml.net and follow the directions. Don't forget to visit the Pentax Users' Gallery at http://pug.komkon.org .
