I highly recommend InsanitySorrow's http://tesalliance.org/forums/index.php?/forum/118-graphic-artistry/ over at TES Alliance. You can work at your own pace, and they're written with the beginner in mind to cover all the basics that it sounds like you're looking for. And he's around for support and to help with any questions you might have.
If you want to fully learn the program, whether its gimp or photoshop, then you should sign up at digital-tutors.com
go check out some of their free videos. Some of their photoshop courses are almost 90 hours long.
45 bucks for one month. totally worth it. they break down every inch of pretty much every program there is. I learned how to use everything about mudbox in a few hours.