could you explain why it may be anoying for some users.
This is a pet peeve of mine. disabling me from stealing your image isn't going to stop me, and I am not that technically advanced! As was mentioned, I can get to the source code anyway, and once I do, your image is mine!
Why does it annoy me so much?
Number 1 because if I want to copy anything, like the title of your page to link to it or get to your source code to copy your meta description again for linking purposes it takes me twice as long!
And Number 2 - if you have a form and I want to paste in some info for you or perhaps copy my comment before sending, I can't do it til I look up the darn keyboard shortcuts. (I realize this probably doesn't apply to your page)
Number three...why do you a) assume I'm a thief and b) assume I'm a stupid thief that goes around the internet trying to steal images without knowing how to overcome a silly (hahaha) non-effectual javascript?
/rant