Bear's right, Print Screen will do it, but that just puts the pic in your clipboard, you must then exit the game and paste it into paint or similar to do anything with it, a bit labourious
There is a CFS3 keypress which stores a screenshot in one of the CFS3 folders, but to be honest your best bet is to use something like Irfanview.
http://www.irfanview.com/Irfanview is a really good, free, bit of software that lets you view and manipulate all kinds of images. It also has a capture screenshot function. Install it, run it, and then go to to Options
Choose the capture screenshot option, and then configure it as per the screen here. Select a keyboard shortcut and file settings and then click start.
Once it is running just start CFS3 and hit the hot key to your hearts content, all the files will be in your folder once you are done.
It's good because you can use it for all sorts of image capture on the PC. I used it to capture these screenshots, it took seconds!