Here you can find the complete series of articles I wrote on how to photo-edit with the free photo-editing program GIMP. I am not being paid to endorse GIMP--I just really like it!
These tutorials take you from the basic skills (such as how to crop images) to more advanced techniques (such as how to use layer masks) and include several unusual photo-editing techniques (including how to create GIF animations). These tutorials do build on one another, so keep that in mind if you skip immediately to an advanced tutorial.
A GIF animation of a frog I found in London.
Article 1: Easy Photography Editing in GIMP
Article 2: How to Crop in GIMP
Article 3: How to Control Saturation in GIMP
Article 4: How to Control Contrast and Lightness in GIMP
Article 5: How to Change Color Photos into Black and White in GIMP
Article 6: How to Level a Horizon in GIMP
Article 7: How to Correct a Skewed Perspective in GIMP
Article 8: How to Square All of Your Corners in GIMP
Article 9: How to Adjust Curves in GIMP
Article 10: How to Adjust Color-Specific Saturation in GIMP
Article 11: How to Blur a Background in GIMP
Article 12: How to Create Stunning Landscape Photos in GIMP
Article 13: How to Control Layer Masks in GIMP
Article 14: How to Create GIF Animations in GIMP
Bonus Article: A Review of UFRAW (a plug-in for GIMP for photo-editing RAW photos)