Well, you CAN render (cut an item out) yourself, but its just very tricky to master. The tool you are looking for is called the Paths Tool in GIMP. How you use it is click around the image. Each click makes another anchour, which is to say a point that your selection follows.
Holding down the click and dragging means you'll draw it instead. Connect it around the picture you are rendering. It takes a LOT of practice to get it right, but do zoom in.
Then when it connects around to the starting point it'll turn into the dashed lines you'll have seen when selecting stuff before. Just cut that (Ctrl + C or right click, copy), delete the undeded background, paste. Now its a new tempoary layer.
You can just merge it down onto the layer underneath at this point, but its a better idea to go up to Layers, Create New Layer. A new layer will be added, shaping itself to the render. Now you can move it around, rezise and play with the render without effecting everything else!
There is really not many times when you really do need to merge items togeather, since this is done when you save the file as an image. Just keep that in mind
Don't forget, if you know the name of what you are trying to do, there is a lot of guides out there you can google which will have images to follow