Hello Paul, you have the option to set the logo for each organization. Can you help me understand why you need to set a background image beside the logo ? Have you considered how this background image would appear on a mobile device ?
Paul, at the moment we don't have a way to set an image. If you go to the FormLogin page, and view the page's source, you can see that the background section is left blank. Login Style controls the colors as you may have noticed.