Here's the scenario:
Step 1. Person A sees file X (which may be an image, movie clip or something) on website M, and finds it funny or otherwise interesting. Person A posts a link to file X on forum N (which may be LiveJournal, or a mailing list, or something).
Step 2. Person B, who owns website M, is annoyed that person A has linked directly to file X instead of to a page on which file X is displayed.
Step 3. Person B sets up a redirect or something so that anyone clicking on Person A's link will see not file X, but file Y.
Step 4. But instead of file Y being a simple "This site does not allow direct offsite file linking" graphic, it turns out to be a disgusting image, usually a bizarre photo somehow involving someone's anus. Or perhaps the link is redirected to an entirely different website, such as goatse.cx, which prominently features such a photo.
The problem with this is that the victims of this prank are the innocent readers who click the link, not the Person A who put the link up in the first place. Okay, big guy, so you don't like whatsername over there linking directly to your file. How come I'm the one who's punished for it? Whatsername may even be completely unaware of what you've done, if she doesn't bother to check the link after posting. This is inappropriate and unfair.