I don't think the PC version has a screenshot function built in (I played the Xbox version), but you could hit the PrintScreen button, exit or Alt-Tab out of the game, and paste the image into a graphics program.
This topic is a fairly old, but there are
Screenshot Programs you can use to automatically save screenshots. A couple others that I saved links for but haven't gotten around to trying yet are
Gadwin PrintScreen and
MWSnap, or check out some listed
here.
