Shameless plug ahead:

If you speak Portuguese and want to learn how to use the GIMP like the pros, I strongly recommend you check out my good friend Antônio “LedStyle” Cláudio's new project! “LedStyle” and I have worked together before doing podcasts for the Brazilian Ubuntu Planet, and my respect and admiration for him has never stopped growing!  I downloaded the first video (out of 10, plus all the exercise material freely available) and was very impressed by his didactic skills!