Another way to make a background image responsive based on images that will be uploaded is to us inline styles. Basically, you add full <style></style>
elements to the page that are dynamically generated.
I don’t have example code that I can share. Basically what you need to do is read through your available image sizes and generate @media
CSS for each screen size you want to use and use the element ID and set the background image at for the media selector.