It is possible to add custom default virtual background images to jitsi. The default folder is /usr/share/jitsi-meet/images/virtual-backgrounds/.
There are 7 Backgrounds that can be replaced but the name must be the same, because the name is hard-coded in jitsi-meet. So we can add 7 Backgrounds in our multi-domain-setup by adding it to the .../images/virtual-background/ folder for each domain.
The names are background-[1-7].jpg
Maybe @lisa.kostrzewa can provide us with 7 Backgrounds we can add es default for fairmeeting.