There is no possible way. The way that the web works is that the image is copied to the end-user's machine, then displayed in the browser. You cannot defeat this and still have the images available in a web browser. All you can do is make it more difficult to get to the standard options in the web browser, but anyone with any technical skill can defeat that.
Depending on the type of site/images you have, there is always the "if you can't beat 'em, join 'em" approach. By this I mean adding a small, discrete reference of your URL on the pictures. This can drive a surprising amount of traffic in the right situations.
(not that I agree with people who pilfer images, but maybe 90% of the people online don't even know what "copyright" means.... you can always have lawyers explain the term to them, but that might be better saved for issues such as pagejacking)
Isn't there some sort of "watermark" tracking system where you can see if others have pinched your images and displayed them on their sites? Could have sworn I've seen this somewhere before. Perhaps it's a specialized company that offers this service.