What's complicated is that there are three things influecing the screenshot:
Your personal in game brightness/contrast/gamma settings gives you the idea how the screenshot should look like, but the screen is captured without the modifications.
When you view the screenshot in Windows, your personal Windows brightness/contrast/gamma settings are used. On some computers this could even make the editing of the picture uneccessary.
Then there is the picture editing. You use a software to permanently change the pictures brightness/contrast/gamma settings to make them look good with your Windows settings.
Summary:
Don't edit the pictures too much, as they look different on every computer. Somewhere they are still too dark, and somewhere they are already too bright.
PS: I remember in one of my graphic card packages was a demo software with a grey colored plastic card included. The idea behind it was, that every user on the world should set his windows display settings to values, that a selected grey looked exactly like the plastic card color. But this also showed that many monitors are of bad contrast quality, so if the grey was ok, you still couldn't see much on the display.
So it's just impossible that you see any photo/picture on the internet like the author sees them himself.
EDIT: Here's a link how to calibrate your Windows settings for example:
http://www.albumart.com/textfiles/MonitorCalibration.htm(Well, with the settings I like to use, the first test picture isn't working at all)