The only surefire way is to not put the pictures online.
I was thinking the same thing, but refrained from saying it.
The problem with the JS script, is of course that javascript is easily disabled. Also there are plenty of ways to work around it- it doesn't stop you using the context menu key on the keyboard after selecting the image with the mouse, for example. But I've always felt that they were just so silly. I mean here you have an image- in plain site and easily able to be snagged with print screen or another screen capture tool, and all you've done is eliminated one of the countless ways to get that picture. Thing is, if you can see the image in the browser- it's already on the machine. With Firefox, and I'm sure other browsers have this feature- you can view the image info for a page and see all the images that are on the page; as well as the location they are saved on the local drive. Simply copying that file constitutes the same as using the "save Image as" feature.
The only "sure way" is to not put them up; if they are something you sell for livelihood, like- um... textures or something, you could watermark them with something like "SAMPLE", and then only put those images on the page. Or, make them a much lower resolution then the final product. This wouldn't stop them from using it, of course, but it would look pretty crap. (and really, you ought to be making separate images anyway- a 72dpi for the screen/web, optimized for size, and the full sized image.
of course in most circumstances where I've seen this being attempted the images are just the standard everyday images you would put anywhere. Sometimes they are even clipart. If you created the images and simply don't want others to steal them, and don't want to clutter your graphics with watermarks (which would make it look silly)- aside from the obvious idea of not putting them up to begin with, you could always try some steganography and embed something that proves you made the image into it; but again outside the domain of requiring your livelihood from selling the content of those images, that isn't very useful. After all, the only time that steganography would be any use is if you pursued litigation against somebody who had copied it.
