From MacTipper.com. I would imagine that this solution is the same in iChat. I don't use iChat so I can't test it for you.
Leopard Only: How to Remove Photo Booth Backdrops
This is something Apple doesn't make very clear. However, it's kind obvious once you think of it. To remove a Photo Booth backdrop (if you use them) all you have to do is select the backdrop and hit Cmd-Delete. However, you don't have to do this because you can just drag in a picture/movie over the one you don't want.
Also, if you're taking a picture with a custom background in it, you can reset the pseudo-green-screen by hitting Cmd-Shift-R.