Nice!
I use gimp all the time (on my mac) since it's free and photoshop isn't. If you have any questions about how to do something feel free to ask me. Also, check here:
http://docs.gimp.org/en/
If you'd like a suggestion on your cover (it is very nice, this is just a suggestion) I would mirror-image one of the trebble clefs to make the whole thing a tad more symmetric. It would still give the musical flavor. I would probably first try flipping the one on the left so that the bottom of the stem doesn't cover up the insect. If one of them is in a layer by itself, use (
http://docs.gimp.org/en/gimp-layer-flip-horizontal.html)
HTH.