If you can I'd recommend Adobe Lightroom for retouching screenshots. You can adjust the brightness, contrast, lightning, etc. to your willing. That could be an alternative to using Gimp.
I use Lightroom a lot for screenshot editing especially to make them look realistic and Arma III official screenshot alike.
Adobe Lightroom isn't free however and is mostly used by semi- to professional photographers...
You can check out some examples at my Steam Artwork-page, as all of the pictures are edited:
I would recommend it to make splendid screenshots make more splendid :D