I think the most important thing for a screenshot is that it can show why warzone is better than the rest. So try figuring out what makes warzone special first. Afterwards think of how you could make that clear in a single screenshot. And finally, make your screenshot look cool => No empty backgrounds, boring parts,...
for me, artillery is what makes warzone special
I could show that be making a screenshot with lots of flying rockets this could be IN AIR , AT IMPACT , AT LAUNCH , ...
work out the details: how much artillery would i need, how should i position the camera, where should i geographically take the screenshot (rocky's, city,.. + cliffs or flat terrain, precollapse structures or not, should the shot include a base or some units to fill up the background,...)
Or maybe you should just play warzone and when you come along something that looks really good just try capturing it the best way possible.