IIRC, F2 does the job, and it saves your pics in the TGAs folder (you don't need to trawl through subfolders to find it, it's in the same folder as you find the .exe, etc).

It should allow you to take multiple screenshots at the resolution either the campaign map or battle map is at (respectively). You can do what you want with them later, no need to keep exiting and pasting into Photoshop or wherever each time you take a screenie.