I have Gimp tutorials here for basic stuff:
http://techn0manc3r.blogspot.com/, scroll way down and look at the prvious posts on the left sidebar, they have their own section just above the monthly archive. In the
final I have links to more all over the web.
I really need to kick myself into gear; I've been meaning to add to these and do some on equally useful Linux image tools such as Inkscape, POVray, Blender, and Cinepaint.