I've discussed this in my tutorial, I think...but if not..
When you paste a new image, it's pasted as a floating layer, which, as I see it (though I'm no GIMP geek) is pretty worthless. So when you paste it, either click Layer>Anchor Layer to make it part of the bottom layer (usually not a good idea, especially if you want to scale it) or click Layer>New Layer to make it it's own layer. Then, get the scale tool out (Ctrl+T i think is the key bind) and go from there.