You can also use steam and "add non steam game" to the library, then run it through steam, and still be able to take screenshots as long as the in-game overlay is enabled. That's what I usually do, but I almost always forget to when I start playing, haha.
Edit: just had a great defensive game on Arica Harbor in rush mode.
I was in the machine gunners seat, jumping in and out repairing the tank (yes, there was a driver, a good one for once), keeping it alive, then shooting everything that moved. Surprisingly, no "Hax" accusations that round.