Steam has a built in function for ScreenShots and I think the default key is F12.
In any case you should head into your game and press Shift-Tab, this brings the ingame Steam menu where you can configure the key for screenshots and where they will be saved. Just pic a location on your hard drive that is convenient for you to access and try it.
You should hear the sound of a camera when you press F12 in game. That lets you know steam has taken and saved the screeshot.
Once you have you screenShots taken, you can host them on a site such as IMGUR or Photobucket and from there you can post the url and share them with ease and speed. I like IMGUR because its really fast to use. I can hold my breath take a screenshot and have it posted on the forum before I run out of air. See Ill try right now...
I could almost have done it twice with one breath