A GIMP 2.8 Quick Tutorial
I was perusing my zebra photos one day and came across one showing a zebra running away from me at full tilt. The photo was really underexposed, but I liked the dynamics of the image, so I loaded it into GIMP and started playing around with it.
What follows is a description of how I created this image:
from this image:
I used the Rectangle Select Tool  to crop the photo. I tried a couple of variations until I settled on this roughly Rule of Thirds  version.
To brighten up the image, I selected Colors > Levels...  and pushed the Auto button . Sometimes automatic level adjustment results in a nice image; other times it can make some of the colors look funky. In this case I was generally happy with the result, but the image still looked rather dark and dull.
I decided to make some manual adjustments, so I selected Colors > Brightness-Contrast...  and turned both brightness and contrast up by 20 .
Turning contrast up darkens the picture, which is why you usually have to turn brightness up at the same time, while being careful not to turn brightness up so much that it blows out the light colors in the image. The two adjustments do not have to be equal. You need to use your artistic eye to decide what looks best to you.
The grass still looked pretty dull, so I selected Colors > Hue-Saturation...  and turned saturation up by 50 .
Saturation is another adjustment determined by your artistic eye. I tend to like slightly strong colors, but not so strong that they look unnatural.
I'm very happy with the final image, which is quite an improvement over the original shot, and just goes to show that you shouldn't necessarily toss any underexposed photographs.