The best way to do this is to properly unwrap the car, then paint the image onto it inside the 3d viewport. I explain how to do this in this post:
Just add the texture to the texture setting instead of the texture mask setting in order to get the picture’s color in your stencil.
If it is only going to be seen from one side, you could also look at it from the side and unwrap it using project from view. The picture won’t look distorted, but it’s not suitable for when it needs to be viewed from other angles.