I'm not sure if there is a way to do it in game. Since I don't use the printscreen key. I use ImageShacks Screenshot Uploader. It has an ingame thing built in, that takes the screenshot and automatically uploads it to my account. Very convient for me.
As far as pics being too dark.. yea I have the same problem. I usually keep Gamma to max so I can see everything. But after taking screenshots, everything is dark. So the only thing I do is use google's Picasa program. Photoshop is probably overdoing it for game screenshots.
Screenshots are grabbed from video card memory which i believe is before any gamma settings are applied. Once you take a screenshot, use an image editing program to adjust the contrast/brightness.