CA has always used the same system for screenshots in all Total War titles.
You press a key, F2 or Printscreen, and a .tga image is stored in the folder
"MainGameFolder\tgas\".
The tga format isn't that unusual either. It can be opened by most graphic editors and viewers. Here's a link to Irfanview, one of the most popular free viewers. http://www.irfanview.net/
With Irfanview you can view tga's and convert one or many files to any other format you like, for example jpg. If you take a lot of screens, this will considerably lessen the space needed to store them.
Using Fraps may lessen your performance and alt-tabbing out of the game to insert the content of the clipboard is just tedious and may lead to ctd.

R'as