Jackson,
There are three parts to successfully reskinning an asset. The first is to understand and have a basic mastery of the tool one chooses to use, in your case, GIMP (which is also my preferred image manipulation tool). SInce I don't know how much you know about GIMP, I'll answer in the general, and you can take what you need from this. An inexpensive, good place to start, is with the WIKIBOOK
GIMP, at <
http://en.wikibooks.org/wiki/GIMP>. Unfortunately, as I look at the contents of the book, it is not very complete. There are also video tutorials on the GIMP site itself, at <
http://www.gimp.org/tutorials/>, YouTube, and likely other video hosting sites, too. And there's and email list for users, to which you can subscribe at <
https://mail.gnome.org/mailman/listinfo/gimp-user-list>. There are printed books (many with on-line editions), too, if you want to spend a bit of money. I'm a big fan of Akkana Peck's book, GIMP,
From Novice to Professional, but you might want to old off on that, as there's a new edition, covering the current version of GIMP, which is scheduled to release in mid-September.
The second part of successful reskinning is knowing how the skin is applied. This is easier with some things than with others, but using a tool like Asset-X will help you understand how the skin is applied, so you can change it as you need.
Once you've finished those two, the third thing you need is practice.
