To be honest, I'd rather see art in the same style as the game cover rather than a screenshot of the game in a Steam card. Screenshots kinda feel cheap to me.
I like a lot of your ideas for a card, just downscaled a bit so it would fit on the card itself, and with the game cover art style. Here's some of my ideas.
On Patrol- 2 Greycollars walking in the mud, tense aware. A jeep can be seen in the background.
Rolling Out- Front view of a Greenbelts tank in the middle of a battle, hopefully with some drawings on it. Think the detailed version of this:
http://steamcommunity.com/sharedfiles/f ... =240597424Resistance- A Brownpant is looking over the war torn town square below, occupied by a Greenbelt Humvee.
Medic!- A Brownpants squad is pinned down on a bridge. One of them is wounded and in the open. The squad suppresses fire while someone makes his way to him.