I agree with Atmo, it's always much better to use a graphics program and do it yourself, you're much freer, and you will inevitably create a much better and much more personally rewarding skin.
But there are a couple of tools to make the task less ardous. The link Mr. Jones has given you is good.
I would definitely recommend the "wizard" program in that link. You do your overall design on the Main.bmp (the base of your main window), and then use the wizard to extract the excact parts it needs into the other files (such as the play/pause etc. button file, or the shuffle/repeat button file etc.) You can then edit those files, knowing that they will be perfectly lined up with the base. If you don't know what I mean, you will soon enough once you start skinning.
Also, once you've completely finished your skin, it's worthwhile getting the "decreaser" program from that link as well. It makes your skin smaller (Kilobytes-wise) without at all sacrificing any picture quality. It works for most skins, but I had a problem with a couple, so make a backup of your skin first, just in case it doesn't work. If it works, it should be fine. I made one of my skins go from about 205Kb to 146Kb. This is good, because often people will be discouraged from downloading your skin if they see that it is too large. If your skin is a picture skin, it will be quite large. It will also have a larger percentage cut from it when decreased.