New WP user: media credit for BACKGROUND image possible?

I’m a total neophyte, so please forgive my potentially stupid question, but I can’t seem to find any instructions for adding a media credit (in this case, a photo credit) to a BACKGROUND image—i.e., to credit an image used as the “wallpaper” that sits behind individual posts.

I’m just beginning to create a companion website for a small community newspaper, and I want to use a recognizably local image as this background.

I downloaded and activated the Media Credit plugin, but it looks like this plugin only attaches credits to media files contained within posts.

this is the site, so far:
https://www.macopolitan.org/

many thanks for any advice.
HM