Steam has just been updated to allow screenshots. The new feature just implemented can be accessed by simply pressing one button while playing any game on Steam. This will allow you to take screenshots of any game that is running the Steam overlay. You can then manage your screenshots from the manager section, and publish them to your Steam profile, or other social networks like Facebook or Twitter to share with friends.
The screenshots are stored on your own personal Cloud storage within the service up to 1GB in size. By default, the client sets up your default screengrab as f12 but can be changed to whatever you like by accessing the settings tab while in Steam. You can also manage your screenshots in game, by pressing TAB SHIFT and pulling up the menu, where the screenshots tab has been newly added.
It’ll download automatically the next time you log in to Steam.