I am not sure exactly what you are doing, but your post caught my attention.
I don't know what version of Moodle you are using or what theme. However, I gave a presentation at MoodleMoot in 2017 that might provide you some insight. I use Boost. Yes, I also use a "background" image. There are probably countless ways to do what you want to do. I define the location of my background image in my Preset file, but the image is actually on my website. This is how I do it, but it might not be "standard."
Anyway, here is a link to my presentation and some resources.