I recently stumbled on this very nice and informative videopodcast called "Meet the GIMP!".
As its title suggests, it's all about showing the potential of this free and open-source graphics software while focusing as well on some theoretical aspects of image manipulation (what do the blend modes like "Multiply", "Screen", etc. actually do) as on the practical side of using this software.
The presenter is Rolf Steinort who in "real life" is a teacher for biology and chemistry. His profession shows in the videopodcast, too, in a very positive way.
Although working without a script, his shows are structured, build upon one another and keep a moderate pace so everyone can follow. He doesn't reveal any super-secret magical tricks but explains the fundaments of good digital image post-processing, e. g. using layer masks, dodging/burning, etc.
So even if you are fairly familiar with image editing or even with the Gimp itself, "Meet the GIMP" is a relaxed and enjoyable format.
All in all Rolf Steinort is doing a really sympathetic and informative show there that deserves even more attention than it is getting already.
He tries to put out one episode each week and there are 32 episodes so far with an average running time of about half an hour, although sometimes it's 20 minutes, sometimes 50.
So if you feel like it, give it a try.