CCS 3 will offer this possibility, but it's still not cross-browser and you cannot do it with traditional HTML + CSS... yet.
Websites having a tilted image do it by rotating it in, say, Photoshop and making its background transparent. That's the whole trick there's to it.
Tip: save that picture to your HD and see by yourself. That's probably just an squared image with transparent background, or maybe it has the current background cut nicely to fit there.