Let me explain in more detail.
If a user goes to Rotten.com and browses from their web pages, each time an IMG tag is accessed and the image is loaded, the rotten.com server can tell that the tag was on a page served from its own server, and will present the image.
When a rotten.com image is posted to a thread here, when the rotten.com server gets the request for the IMG, it can tell from the http headers that the requesting page was not a rotten.com page. It can also tell that the requesting page was a Free Republic thread.
Many sites use this ability to tell what page generated the request for an image in order to substitute another image, so that people don't post their images and use their bandwidth. Angelfire is an example of this.
Rotten.com has this habit of changing images posted to FR to be something particularly nasty. You don't have to believe me, plenty of others have seen it before.
A user's cache comes in to play because it can make it so that a user does not notice that the image has been swapped on him. If the user had viewed the correct image at rotten.com, and then goes to post it here, when the user gets the preview page it can (depending on browser settings) just show him the cached image for that URL, rather than going to the server to get the image served again. Everyone else would get whatever the server delivers- which has been "Tub Girl", it has been some gay porn, it has been some decaying bodies.
I hope you have a great New Year, Glenn.
Why?
Okay, admittedly, I don't understand how these things work, but I would think that they would WANT the hits on their site....or does looking at a picture on their site not constitute a hit?