I think your red tank and blue tank need a bit more contrast.
The grey tank and the green tank seem to have a bit of banding, but I'm not sure if that's always a bad thing.
I can't crit the animation, since it's not animated.
Otherwise, though, I think they look pretty good. The BG you did is pretty nice, too; the trees on the right side look unfinished, but everything else there is pretty nice.
I really like how the yellow one looks like it's converted farm equipment.
The grey tank and the green tank seem to have a bit of banding, but I'm not sure if that's always a bad thing.
I can't crit the animation, since it's not animated.
Otherwise, though, I think they look pretty good. The BG you did is pretty nice, too; the trees on the right side look unfinished, but everything else there is pretty nice.
I really like how the yellow one looks like it's converted farm equipment.