To make sure it is rendered press F10 on your keyboard, then look for a button that says "do composite" make sure it is on.
Also make sure you have a composite node, you add those by pressing SHIFT+A>output>composite.
Hope this helped.
To make the image look more realistic, i would use a higher res image and make sure that the render is not rendered in a bigger resolution than the image.
Compared to the image, your car is super clear. You should either get a clearer image, or add some depth of field maybe so that the car is in focus the rest isnt. It COULD work, not sure though.