This post has nothing to do whatsoever with A:S:S... I just felt like making a tutorial. Why am I making a tutorial? Well... look at this picture above. Look at it.
See what I mean? People... listen to me very carefully now. There is NO point in using a greenscreen for your picture, if you're going to leave little bits of green in it. It just looks weird, ok?
So, here's now you do this right! (I am sure there are lots of other ways to do it right too, but this is the way I've found fastest and easiest.)