See these sites.
https://devblogs.microsoft.com/oldnewthing/20070302-00/?p=27783
https://www.itprotoday.com/microsoft-windows/how-can-i-configure-default-settings-for-new-users-
https://woshub.com/setting-desktop-wallpapers-background-using-group-policy/
If group policy is not an option you can use a script that runs at user logon.
https://superuser.com/questions/277805/how-do-i-set-the-desktop-background-on-windows-from-a-script
https://www.joseespitia.com/2017/09/15/set-wallpaper-powershell-function/