I was at B&N, and they had a copy of a The Gathering Storm, with Robert Jordans signature in it. But the book was released 2 or 3 years after his death. Did he pre-sign these books before his death? What gives?

I have the same thing, but it's just an image of his signature, not his actual signature.

That was my assumption, but when I asked, I was told this other theory.