Sorry, you can't totally override the users preferences in Firefox from a website. Right-click and Save or Copy isn't the only way a user can save an image from a web page, they can open the Cache and save it from there. Or they can save it from Page Info.

This is a security issue for users, for every "good" reason for blocking contextual menus, there is the possibility that it could be used for nefarious purposes.

Try placing a transparent image over the image you want to protect so that "Save Image As..." will only be able to save the transparent image. I am seeing more and more web sites doing it that way to stop users from saving their images.

Many Thanks I'll try the transparent way

If you place a transparent image on top of an image then it is still easy to save the image via Tools > Page Info > Media.
A better way to do this is to use a Flash object (or Java applet, but Flash is more common) to display the images and retrieve the images via a secure connection in that Flash player if you want to protect them.

This really doesn't work with Firefox: Tools > Options > Content : JavaScript > Advanced > Allow Scripts to: [X] "Disable or replace context menus"