A popular trick to prevent image theft is to "brand", or "watermark" them by putting your URL or logo into the image. Anyone who takes your images and posts them somewhere else will at the same time be advertising for your site.
There are a plethora of Javascript tricks that "prevent" the user from right-clicking on images and selecting Save As.., and so far none of them have worked, or worked in only one browser. These scripts can be defeated because they cannot cover all the entry points to the image. The browser belongs to the user, they can do whatever they please. Even, if by some miracle, a method was found to block the user from saving the image through the browser, there is still the unstoppable resource of the cache.
The rule of thumb is that if an image can be viewed on the users computer, there's nothing you can do technically to stop them copying your images, legally you may have a few options.
The second rule of thumb is that if your images are that important to you, don't put them on the net.
