A barefoot man in an orange shirt, shorts, and a baseball cap is sitting on the open tailgate of a silver pickup truck.
A man is sitting on a car.
The man is sitting on the tailgate. The word "car" can mean truck.
A pickup truck can be described as a car. The man is sitting on the tailgate, which is outside the passenger cabin. This means he is sitting "on" rather than "in" the car.
A truck is a car.