Posted on: May 25, 2018, 11:09:23 AM
That's a field that you would hardly ever use. The background image would show below the entire module, and the size is just a way to stretch the image. I would use the setting above that labeled "Background Image Size Option", it's the same thing but with predefined settings; I typically just use "cover". The custom field would be if you want the image to be 200% of the available size or something like that.