Try this - http://gimps.de/en/tutorials/gimp/pictu ... /index.htm
#1 create a brightening layer
Duplicate the original image layer in the layers dialog. Change the Mode of this duplicated layer to Value -mode to use this layer for a retouching brightening of colors. Further editing of the image, without this masking layer , could lead to a change of original hue and colors within the picture.
#2 improve brightness
Apply the Color » Levels -dialog to the retouching layer for manual or automatic correction of brightness, color and contrast. Click on the Auto Correction -Button in the levels Dialog to try Gimp's automatic correction of brightness, color and contrast.
#3 check color change
Switch off the visibility of the first original layer of the image to check the change or difference in colors of the retouching layer.
If the hue and colors of the original layer affect the image badly you can just delete the original layer.
Otherwise combine all layers in the image or try the manual brightness correction, if the automatic correction did not work for your image.