Gord does the same type of thing to protect bandwidth costs at TVShowsOnDVD. It's some security coding that makes sure that the picture link is being viewed at the original site, and not deep-linked at someone else's site. It's so you don't pay the hosting bill for a pic that some other web site is displaying.
If the check fails, then instead of showing the picture you intend, the code re-routes the request to show a different pic (usually one that says "hey, if you want to see this pic, please come to my site to view it").
You'll see more and more of that from other websites as time goes on. Guys like CNN can afford to be generous with there website hosting bills. Little people who are not funded by conglomerates' deep pockets cannot!