Currently Steam screenshots are in beta-testing (you can also participate). This allows players take screenshots and share them from any game that uses Steam overlay.

If you’d like to test the feature, you can opt-in to the latest Steam client beta by going to Steam > Settings > Account then changing your beta participation to “Beta Update”.

Once you’ve signed up for the beta, press F12 in any game to take a screenshot. Screenshots are automatically uploaded to the Steam Cloud after exiting the game, or can be manually uploaded while playing through the in-game overlay. Screenshots can be managed through the Steam Community web site and can be published to your profile to share with your friends. Games that use DirectX 8 and earlier are not yet supported.

You can adjust the screenshot hotkey by visiting the In-Game section of Steam settings. If you prefer to not have your screenshots automatically upload to the Steam Cloud, you can set this in the Downloads + Cloud section of settings. Currently in the works is an interface that will allow you to manage your local collection of screenshots and choose which ones to delete or upload to the cloud manually.

The feature is still a work-in-progress while in beta and they will be fine-tuning it over the next couple of weeks. Please visit the Steam Screenshots thread in the Steam Forums to point out bugs, suggest improvements or provide general feedback.

Source: Steam news