Add shadows. You know, it looks like these things are flying in the air, which - although of course I can only make assumptions - is not what they are supposed to do. If they, however, are supposed to fly, then please disregard that comment.

These ... well, skidmarks or whatever you call these trails this tank is leaving - they seem to be missing on some of the shots (i.e. the fifth, on the right). Maybe the tank isn't touching the ground there, so maybe that's okay.

I agree with Michael on the emptiness, but if that's what you want, then go ahead. I don't like the lensflares as well, but I can't really tell why. Maybe try making them more transparent? I don't know.

Whats with the first screen? It's so dark that I can't see anything on it, aside from the rough outline of the tank (it looks good) and I think there are some stones in there, maybe even a tree - no, wait - three! - or at least something that resembles that in the distance. You know, some particlestars would have been nice. But who knows, maybe the sky is covered in clouds, I don't know. I can't see it, at least.

All in all, they are good. Just add some shadows. You won't make it less empty anyway, and the other complaint of mine was just a minor issue that's likely not really noticeable when the tank moves.