I would imagine that this is almost impossible. The image has to be downloaded to the client machine for the bowser to render it on the screen. If its on the clients machine then the user/visitor can save the image, either from the browser, or from the cache.
There are various tricks that you can use, setting the page up so that it doesn't cache, or using onMouse over techniques to change the image/lose focus, change the page - these can all be got around by viewing the source of the page, copying the url of the image and then loading that directly.
The only sensible answer, would be use poor quality images, shrunk images, images with "sample" stamped across them.
It *might* be possible to use Java to do this. Browsers that don't support Java would then not view any images.
The following was submitted by Dave Zimmerman
You can disable right click, hide the source with frames, scripts, you can put the image in an applet, but in the end you will be foiled. The hardest to code would be the applet but all the viewer has to do to get the image is to do a screen capture and import it into their paint program and viola, they've got an exact copy of your image. The only thing you can really do is put copyright information on your site and watermark your images. The latter if done right doesn't marr the image too much.