Probably the best I can do is suggest you practice. I started texturing in GIMP a year or two ago, and it was really weird (not just because it was GIMP, but because I had no clue what I was doing, I would have sucked just as badly with Photoshop). But as I used it more and more I got used to painting textures and my work has improved.
One thing you might like to look at:
http://www.lowpolycoop.com/2006/03/polyglog-01-trial-run.htmlHis (and everyone else at the co-op) texture work always astounds me, and it's cool to get a look at the process (especially since it's explaining what he's
doing rather than "use Photoshop filter X, followed by...").
Another thing you might consider is posting your work there. If you're already going to release your source for the purposes of OA, why not the LPC as well? I think everyone uses Creative Commons license there, but I don't think they'll mind if you use GPL instead. Have a look around, anyway