Hey Reynolds, I just remembered you could have downloaded GIMP from just about anywhere...
Schatzi and Reynolds, try this website...This is where I downloaded and I kept going back. Sorry for the confusion. Lots of good gouge on this site. Honestly though, when I got stumped with something, I would just do a google search for "using gimp to [insert your effect here]". Lots of blogs and stuff for gimp. A good many of these tutorials apply. Concentrate on the layers, selections, lines, paths, etc. I'm not sure I have the patience to put a tutorial together for AH skins, but I'll be more than happy to help with specific questions on what tools to use for specific effects.
And Reynolds, part of the problem with that mouth is probably the way the model wraps the bitmap file. I've had a lot of problems with my jug skin just getting the individual skin sections to match up. Lots of trial and error. I'll see if I can't maybe put some screen shots together to post to demonstrate how I try and line up the different sections. If you look at the last post on Puck's LA, he has a screen shot showing how he tried to figure out the way the model wraps the skin sections...
http://www.gimp.org/http://www.gimp.org/tutorials/