This guy seems to cover everything a beginner needs to know and he has more advanced stuff as well.
I like that he doesn't edit out his 'oopses' and try to appear perfect. It's much more like real life. It reminds me of Kahn Academy where the guy does the same thing when doing calc or algebra. I think it makes for a better lesson myself.
I can't give you a direct link, because of his use of aspx and how he built his site, so just look down the list on the left for GIMP:http://fotolinq.com/gallery1.aspx
Lots of photography stuff there too, but I didn't delve into that stuff at all.