Looking for a very specific Firefox extension
 

I'm looking for an extension that allows me to redirect the URL of an image (or any other element, text included) to a different URL - So, say as an example I'd be able to have the Gamingforce header banner show up as a gif of a dancing penguin, or I'd be able to change individual bits of body text.

So, basically, I'm looking for adblock with a sense of humour. Is there such a thing?

edit: god damn, I put this in the wrong forum without realising and I feel thick. Someone move this :(