I think you did a fantastic rendering on your characters and the car up front, but you need some contrast between them and the background. The composition is mostly good, (I love the detail of the smoke leading behind to show they are airborne). But the art is like 80% one color. Its hard to distinguish anything but the characters when everything is the near same shade of green.
(Also theres points on the background where shading on the ground doesn't appear to make much sense. Like around the main characters head..)