A banner is a strip above and below each photo with your logo, phone number or a sale message. Upload yours in Settings under Banners.
Then, on a car, choose the banner under the backdrop. It goes on every photo of that car, the ones already made and the ones to come. Banners are free: changing one doesn't use credits.
To put your logo on the wall behind the car or over its license plate, see your logo on the wall and plate.