Yeah the Camaro looks out of place. But look at the height of "Arnold" there, his shoulder width, the size of his head, how the neck attaches to the torso. It looks.. odd. Like Arnold's head was poorly Photoshopped onto someone else's body.
Yeah, the poster would have been much better if they had just left that out. It's a pointless distraction that completely breaks up the atmosphere of the shot. Somehow I doubt his car is going to be a central focus of the film anyway.
Oh, for all we know it's a wacky sidekick car that always making snarky comments about the goings on.