Allow for more than 1 image in the banner ads. This can then be used to accomodate the HTML <picture> syntax which allows for images to be swapped out for different device sizes. Currently (as far as I know) you need to create a whole different banner set for each device if you want to have a different sized banner image for a mobile device. This is a lot harder to manage. This is needed because desktop banner images aren't generally suitable to be displayed on a mobile device.
