While ingame, press shift + insert.
Works with non-steam games too. When you press shift + insert, this is what comes up

Then when you exit out of your game...

You can then publish your screenshots, and add a caption.

You can also get the direct link address of the screenshot and don't have to upload it anywhere else.
You have to go to Steam>Settings>Opt in beta to be able to use this
WARNING: You're steam will crash a lot more when you opt in, good news though,
http://forums.steampowered.com/forums/showpost.php?p=20586940&postcount=40