They're trying to prevent using they're site to serve images for a site hosted elsewhere. ie: Aggregate four free accounts to pool the storage by spreading your images around. Quick-and-dirty way to do it is check the HTTP_REFERER field. If it's from a different site, block it. But typing the URL in works.
To test it, I clicked the link and got a 403. Then I hilighted the URL and hit [enter] and got the image.