If you want to implement the right-click-disable, fine. But try not to delude your client(s) into thinking this will stop people from getting the image. If it is displayed on screen... people can and will download it. Dispite any efforts you make. I agree with others in this thread... If they don;t want people to get the image then DO NOT put it on the web. Period, paragraph, end of story.
If they want it they will get it no matter what kind of JScript you write to block specific browser access.
Same goes for CSS, HTML, and linked JS files. Of course with linked JS and CSS you can make dynamic scripts that generate the file to limit the "referral" access and to elude the person trying to access the file.