Retouching an Image in Gimp
Suppose you have an image:

You decide it needs work. Particularly, it's a bit on the drab side. It's too dark and the skin tones are boring.

Almost all of the "photographic" retouching tools can be found under (right-click the image) Image/Colors.... I invite you to experiment with the tools in that submenu. I will not cover all of them.

If you are a photographer, you know that much of retouching is a matter of trial and error. You may be on your way to the right end result but you suddenly loose the hilights or the shadows and then those areas are unrecoverable.

So, using the trial-and-error method, let's see what we can do.

Well, it can stand to be lightenned.

So, we select the Brightness-Contrast tool. Using careful adjustments, we get a result that looks like an improvement.

We seem to still have some color detail. Maybe we can give it a bit of a boost. We'll use the Hue-Saturation tool.

Personally, I like my people to look warm - unless I deliberately want them to look uncomfortable. This chick looks freezing cold. I might be able to warm it up some by using the Color Balance tool. All I need is a bit less blue in the hilights. I click the Hilights button and try adding a bit of yellow. Through experimentation, I find that unsetting the Preserve Luminosity option brings down the hilights nicely. It's a judgment call.

Preserving luminosity. The highlites are still almost too bright, particularly the center of the forhead.
Not preserving luminosity. The highlights - in my judgment - look better.

If only for the sake of introducing another tool, let's steapen the exposure curve - using the Curves tool.

And there is my modified image.

Now, if I start from scratch and begin by lightenning the midtones (using the Filter Pack tool and hitting the Midtones Values) and then follow that with the same steps listed above, I get a bit of a better result.

Here is someone else's work on the same original image:

The above image is retouched by gizabe.