Alright, on the right you should have another window opened [Channels, Paths, Undo...] that shows the different layers. Your stock image that you have, you'll need to right click on its layer and near the bottom click "Add Alpha Channel." After this is done, you can Select All and use the eraser to get rid of the background not needed or you can use the fuzzy tool, then click edit, then cut. Simple enough so far?