When it comes to the background images, I knew I wanted them to be big enough that the player would only be able to see a portion of it based on where they were on the stage. So if you were on the far left of the stage, you would only see the far left of the background and as you went right you would start seeing it move with you until you reached the right edge.
Originally, I thought to just take the backgrounds my artist gave and scale them up to fill the entire play area, but scaling them that much made it so big it was difficult tell what you were even looking at. So a much better solution that I ended up using in game was to have the background images scaled to be a little bigger than the lens of the camera, and have them follow the camera with an offset (parallax).