you need to understand something called Color Levels. The original image is overly underexposed, and the edited image has the brightness and shadows better than in the unedited image. This tutorial teaches you how to rescue such underexposed images and also how to optimize the shadows and highlights in your image.
The Color Level tool in GIMP allows you to adjust how bright or how dark you want your image to be. Many a times, we face this problem of over exposure of whole or portions of an image and we just can’t get it right with a camera (unless you are a pro)! So GIMP to the rescue.