Want to salvage under-exposed photographs? Give Gimp's Auto-Equalize feature a try. It's possible to salvage under-exposed photos by making manual adjustments of brightness, contrast, and etc. But nothing is quick than one click on the auto-equalize function. Take the following steps to auto-equalize your photograph.
Auto-Equalize creates high-key photographs. To keep your photograph low-key, see the "Gimp by Example: Auto-Normalize" article.
The following is an under-exposed photograph. It's under-exposed by at least two stops.
Using the Auto-Equalize functionality of Gimp renders the following correctly exposed photograph.