Ahh alright. Good. That's what I mostly use, too.
Like I said, it's good for your second.
Your cutting is pretty good, just try not to make some spots look so jagged. To soften up the cutting, take the blur brush (in the toolbox - it looks like a little water drop) and go around the entire horse. This makes it look a little more natural against the background.
Also, as you do a couple more you'll learn how to pick background stock and horse stock that work together more. For example, the horse in this case doesn't go too well with the background.
You should google some manip tutorials for gimp for some great advice and methods used by others. That is what helped me improve the most.