"Background" is for environment maps, like HDRI that wrap all around a sphere which encircles the scene. A "backdrop" is just that, a flat backdrop always straight behind the scene facing the camera. It will render at the size of the production frame. You can go in the render room and set the size of the production frame to match the pixel height and width of your backdrop image. I think you are out of luck in seeing the backdrop until you test render, at least in the latest version. It would be a nice feature to aid in compositing on a backdrop or background. Patrick Tuten