I think the Captian America figure should be turned more into the picture, a little more profile. The power of the punch is weakened in the current pose. The change would also featue Captain Britian more. Captain American's shield is where your sending the viewer first. Why? because the shield is large, it has the brightest colors, and the most contrast, you might want to add some shadows. Remember, Size, location, color, contrast, etc., all focus where the viewer looks.