I know the very rough theory on it, but not the practical side (I've only ever done it once for an old friend of mine for our joint hack... Didn't really see the light of day due to bugs elsewhere)
there is one on whack a hack, but it's in spanish, and i don't like reading in spanish (although i can) because there are some words that i don't understand.
It's not too hard though. It's simply replace the image with a new background (repointing if necessary), then fixing the RAW file (located as the screwed up image next to the original pic in unlz (click Save Raw)) using NTME, and reinserting it.