Blueprint / .twig / .md for CSS background image

I would like to be able to select an image in the page admin, which then displays within a template div as a css background-image.

There is an incomplete solution here, which I’ve tried without success: How to have a css background-image from form field?

Can anyone please help with the correct blueprint, twig and md structure?
Many thanks

If anyone needs the answer, I finally found it here: CSS Image Loading
Sorry for wasted reads and thanks to poster(s) of solution.