Prior to realizing I needed to create a child theme, I set a picture for the header banner (i.e., not random). I created a child theme with just style.css, and only added:
... to that file.
When I activated the child theme, the header banner selector was set to random. I was able to set it back to the photo I want.
When I make a preference, like this, where is it stored; in a place related to the child theme?