By disabling the right mouse button, website designers think they are able to protect the HTML source or prevent people from downloading images. Useful functionality is taken away and there are so many ways to circumvent this (for example by disabling JavaScript temporarily) that it is not worth the effort.
There is no way to protect the viewing of HTML source or the downloading of any of the objects part of the page. At the time a visitor sees your page and graphics, those objects are already stored on the computer of the visitor in the cache. Besides, most browsers have the ability to save the page and all graphics to a designated location.