You can find a quick one-liner on how to disable the browser context menu anywhere, but I just wanted to make sure you know about the options.
Disabling the context menu entirely can be a nuisance for many users, as they may depend on it to create a bookmark, navigate back/forward or refresh the page.
If you primary purpose is protecting images from a quick "Save As", the jQuery example below demonstrates how to disable the context menu on images only.
$('img').bind("contextmenu",function(e){ return false; });
If you wish to "protect" the entire page, you might consider use a jQuery context menu, which does not include Image saving capabilities:
http://plugins.jquery.com/project/jqueryContextMenu
http://www.trendskitchens.co.nz/jquery/contextmenu/
Of course, you're never really protected using this method – the user can always disable JavaScript, or print the page. The best level of protection is probably a watermark.