I have programming background with some experience with image manipulation via GIMP and Fireworks. As working with image manipulators is part of my work sometimes I decided to improve my knowledge in this area.

However, when I try looking for something like

GIMP tutorial

what I am able to find are only some tutorials with how to do this logo and that image, how to apply filter and so on.

What I would need is some decent tutorial explaining the basic graphics concepts like layers, channels and so on so I can see what is happening behind the tools used in GIMP.

What should I be looking for? What are possible resources to learn these thigns?