Using GIMP: Correcting and Touching Up Your Images - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 124

Similar to the Brightness-Contrast function, the gradation curve can be used
to brighten or darken an image. Simply move the end points of the curve
upward (for brightening the shadows) or downward (for darkening the
highlights). Again, you can insert control points to make specific color areas
brighter or darker. You can correct the tonality and set the brightness either in
consecutive steps or in one run.
Shadows - Highlights - Correcting backlit
photographs with the gradation curve.
Backlit photographs constitute a difficult situation in photography. The
surfaces of the photographed objects in the foreground appear shaded. In a
contrary but similar manner, photographs taken with a flash are overexposed
in the foreground. As long as there is enough distinguishable image
information available, you are able to improve these photos considerably. In
some programs, you can use a Shadows-Highlights function to improve your
image. For GIMP, there is a plug-in; yet GIMP's Curves function is suitable for
making these corrections. Select the image ship.png from the SampleImages
folder on the DVD.
Figure 2.51
The image before and after the corrections. Two points on the gradation curve are enough to improve the backlit photograph.