... I would like to use direct editing tools in Site Designer ...
The attached (normal) .zip file shows a way that gets close to direct editing. I set up the container with a background image, but left the image as the default one. In the Header code of the Element panel for the container I set the background image as the .avif file.
The above is writing it in CSS (Juan's 'another option') but involves only the file name. All the other settings of the background image are controlled in SD in the normal way.
Frank
Thank you Frank ! I downloaded it, looked at it, I realized I’ll try it.