Lots of people are going to want to pipe in with their opinions on this one, but in the end it'll all basically boil down to one succinct thought that goes something like this:

"If you don't want people copying your images off your site, don't put them online to begin with."

Unfortunately that's the only sure way to make sure they can't be copied.

Oh sure, you'll have people suggest all sorts of crazy ideas to protect images. In the end these ideas just make it more difficult for your innocent users to view your images, while at the same time making those that would steal them just a little more curious about all the loopholes — to the point of actually taking them just to see if they can. You'll get suggestions like:

However, these suggestions will either not work at all, will criminalize innocent users (the no-right-click script — ie, taking away functionality for the sake of the minority), will only work if javascript is enabled, will only work in v4 browsers, or only make it more work for you to do but no less work to take the images.

This all takes me back to that concise comment quoted above.

There are, however, some things to consider when putting images online that can reduce their attraction for thievery.

Just my 2¢,