The r-click disable does nothing but annoy potential viewers (or buyers, as the case may be). The r-click menu has more action options than "Save As" and disabling it so one can't use it is, imo, pretty rude. I think some people with disabilities may also use tools for browsing that are affected by this. Not to mention it's completely ineffective in Firefox browsers beyond the annoying inital 'can't do that' - which I can click out of and ignore.
As someone else said, there is, sadly, no good, polite, effective way (yet) to keep people from taking images off the net, beyond not giving them access to the image until they've paid for it.