Disabling right-click is widely considered bad web design and is plain rude, among popups, pop-unders, forced window resizing (my pet peeve) and numerous other annoyances.
Even if you install a script to disable right-click, it's trivial to circumvent by going to Tools/Options/Content/Disable Javascript in Firefox, similarly easy in IE. Flash sites can be screencapped - but such sites are a whole different user interface nightmare.
If someone wants to "misappropriate" your photos, they will do so regardless of javascript or Flash tricks. You risk alienating a good portion of your visitors to protect web-sized photos. As said, many users use right-click for bookmarking and other uses.
Panopeeper, you can right-click on the background of a web page in FF, and there is a "back" option in the popup.
edit: Ed, I shouldn't tell you this, but your right-click blocker doesn't work.