There's another trick whereby the browser displays a transparent image over the real one, and all right-clicking and using 'save as' does is save the transparent image. There's always a way around, though, if someone is determined enough. If you absolutely have to post, the safest way is to post a low-res version, 72 dpi and 640 x 480 (say), and keep your >2000 dpi large version to yourself or distributed only as hard copy prints.