As already mentioned, disabling right-mouse click will only stop the inexperienced user. If your browser can display a photo, then you can download it - even without using right mouse click (if you know where to look).
I think the loss of functionality is far worse than the minor benefit one might gain from stopping a couple inexperienced users from downloading your photos.
Better approaches are to use some form of digital watermarking (although many of these can be removed if you know what you are doing), but also to ensure that you only upload relatively low-resolution photos to the web ... even if someone was to download them, they couldn't do high-quality reproductions of them anyway.
A lot of professional photographers have photo galleries on their own websites but limit the resolution of the images they upload - if you want a print or a high quality digital copy, they will send it to you directly ... the stuff on their website is really just a preview.