That cannot be done. Sure there are javascript scripts that try to disable right clicking, but it just doesn't always work. for example, in firefox, the scripts used for internet explorer don't disable the menu, they just pop up the box. Also if a user sees that your site has a javascript script disabling right-clicking, all he or she would have to do would be to disable javascript in his or her browser and bingo, no lock. If all else fails, all he or she would have to do is google image search your site for all images, get the file name and then use wget or something to download it directly.