Stole this from IMDB:

Code: 
 <img src="" onmousedown="return false;" onmousemove="return false;" oncontextmenu="return false;" />
It'll work as long as javascript has access to disable or replace context menus. It does not create any alerts in Firefox, have not tested any other browsers.

//EDIT

See it in action:
Code: 
http://www.imdb.com/media/rm2885532672/rg2675546624
The image is between the <div class="primary"> tags.
xdb Reviewed by xdb on . Disable right click for images (no alert) I spent hours and hours and no good script is found. I am seeking the following: - disable rightclick for images only - works on major browsers including chrome - no alert message what so ever All scripts online have alert messages! and if not it don't work. Rating: 5