A photo before and after my retouch with Gimp.
Author Yoshi Toshibo for Gimps.de with Photo Software Gimp.
The eyes are the most important part of the face.
Eye color of a baby is the most honest on this earth.
Dark eyes often look unattractive, cold, tired and empty.
You can improve each photo, picture, portrait and image of a
face with dark eyes, if you enhance brightness, lightness and
brilliance of the dark eyes with Gimp.
Select the white sclera of each eye with Gimp's selection tool lasso.
Make the selection soft by using the Select » Feather dialog.
I used a value of 5 px to make the selection in my image soft.
Reduce the saturation of previously selected white sclera with
Tools » Colors » Hue + Saturation to make the eyes
more bright, brighter and clearer.
I used the following values to improve the eyes in my image:
Hue: 0 ; Lightness: 60 ; Saturation: -70.
Select the iris of eyes with Gimp's selection tool lasso.
Make the selection soft by using the Select » Feather dialog.
I used a value of 5 px to make the selection in my image soft.
Enhance the brightness and saturation of the iris by using the
Tools » Colors » Hue + Saturation dialog.
I used the following values to improve the brightness in my image:
Hue: 0 ; Lightness: 10 ; Saturation: 35.
The paintbrush 
of the Gimp is a good tool to retouch the dark under-eye bags,
rings, wrinkles, stains and other blemish in an image, picture,
photo or portrait.
Use the paintbrush
to remove, retouch and correct blemish in a photo.
After you have gained more experience you can
retouch photos and images much better.
Create a new transparent layer 
,
Choose white as color to correct, remove and fix dark stains or under-eyes.
Choose black as color to remove, retouch and fix bright stains.
Paint with a paintbrush
on the transparent layer over the spot or blemish that you wish to
correct and retouch.
Blur the retouching layer with Filters » Blur » Gaussian Blur. I used a value of 75x75 px to blur the layer in my image.
Set the layer mode of the retouching layer in the layers dialog
to Overlay to retouch the underlying layer in the image.
Duplicate the original layer, move the copy to the top of all layers. Set the layer mode of the moved layer to Hue to keep the natural hue and colors of the picture. Duplicate the upper layer, set the layer mode of the duplicated layer to Saturation to keep the natural saturation of colors in the picture.