Probably more known to the Linux users, GIMP is an GNU Image Manipulation Program, also available, though not supported officially for Windows. The main thing that got GIMP known is the fact that is a powerful, free alternative to Photoshop. The main disadvantage is that is pretty hard to learn and master, so you’ll need some really good and easy-to-learn tutorials. Apex Web Media Computer Training Software offers some great video training CDs for GIMP.
GIMP Training Videos offers over 12 hours of valuable lessons by accredited expert James Street. The CD / DVD is organized in 16 chapters and includes 110 lessons with all the work files included in the package. All 110 lessons are in audio and also have subtitles. The first 3 chapters with 8 lessons are free and can be viewed online here.
The training videos are compatible with Vista, 98, 2000, NT, XP, Mac OS and OSX.
You will also get your own free certificate (which can be verified online) to prove your credentials.