Anti-hotlinking. Copy/paste the URL into a new tab (so there's no referer).
I know linking directly to image files is in some circles the hip thing to do, but I don't get, as it creates a frustrating dead end if one is interested in more information on the subject that may be on the page with the image or available from following a link on the page.
And, apparently this site admin doesn't like it.
So I stole his image and threw it up on imgur: http://i.imgur.com/Qov1B.png