But, don't forget. Once the image is displayed by the browser, the image is in the client temporary internet files folder for free use. Also, once you pop the javascript alert they know you have disabled the click with javascript. So a user only has to simply disable scripting in IE to do whatever they want. Then there's the "View Source" option so even if the right-click is disabled users can see the image path and get the image directly from the server. Even if you find ways around all this there is always "Alt-Print Screen" and then paste in your favorite graphics program.
I've battled this many times in many ways, but there is no sure fire way around this.
You could always watermark the images. To me that's the most difficult thing to remove from an image.
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