A new automatic Steam client update is now available, and, as noted here, in the new version the Steam screenshots feature is now out of beta and available to all. Here's word:

Steam just made it easier to take and share screenshots of your favorite games.

Press your hotkey (F12 by default) while in any game that runs the Steam Overlay to take screenshots. Then publish them to your Steam Community profile as well as Facebook, Twitter, or Reddit to share them with your friends.

With 1GB of personal Steam Cloud storage, you can upload thousands of screenshots to show off your best moments for all your friends to see. Or you can make them private if you'd rather keep them to yourself.