It’s only useful for bandwidth thieves who host their own HTML but link to your images. Lots of fun to be had by using an image as a 403 error document, too.
Disabling keystrokes is rather pointless if you are concerned about someone copying stuff. The copy has already been made and anyone who intends to keep the copy knows how to do so. It might piss off your visitors if you go about trying to disable browser functionality. In my opinion, capturing keystrokes is only useful for “applications” where you want shortcuts to performing actions - such as Google Keyboard Shortcuts - http://labs.google.com/keys/index.html