Well, even if you could do it - they could always disable javascript, and then grab your images.  I think the more effective way of doing this would be watermarking them.  Might effect the look of the graphic or photo, but that way would make it a little harder to steal it without giving you some sort of credit for it.

For images, you may consider watermarking the images (such as with your url), that way even if someone does download, they have to work at removing the watermark (a lot of poeple wont go through the hassle), or if by chance they do post your images elsewhere, then the images have your url on them, so its almost like free advertising.

Unfortunantly, the only way you can absolutely block this, would make your site unviewable by anyone.

Probably not the best solution to implement long-term.