With flatpak Zoom

Select a custom background image in the Zoom UI.

In the folder: ~/.var/app/us.zoom.Zoom/.zoom/data/VirtualBkgnd_Custom

Find the file with the name {guid}.

Replace the file with a link:

ln -s -f ~/Pictures/Backgrounds/dt-link.jpg {guid}

Use the wallpaper script at login/logoff to change the image before Zoom runs.

See this FreeCodeCamp article for more detail.