Big Thanks for your support! 
we are very close to a solution now, after several more days of searching, and right now i am testing this on a parallel computer with the Manjaro USB installation-stick.
the problem in MATE not showing “user-backgrounds” is not in Lightdm / Slick-Greeter
, but in AccountsService
because it overrules everything:
I changed the permissions of /var/lib/AccountsService/users
- folder with
sudo chmod 777 users
and also all the “user-files” in this folder with
sudo chmod 777 user1
sudo chmod 777 user2
The content of every single user-file is:
[User]
Session=
XSession=mate
Icon=/home/standard/.face
SystemAccount=false
In our Manjaro-MATE implementation the information for the BackgroundFile is missing !!!
so I changed the contents of every user-file to what i saw in your comments on GitHub:
[org.freedesktop.DisplayManager.AccountsService]
BackgroundFile=‘usr/share/backgrounds/mate/nature/Garden.jpg’
[User]
Session=
XSession=mate
Icon=/home/standard/.face
SystemAccount=false
In this case the user has chosen the preinstalled ‘Garden.jpg’ as Background Wallpaper for his desk.
The Slick-Greeter IS SHOWING the user-backgrounds now at the logout-login screen !!!
But this is just a STATIC solution: if a user changes his background, the BackgroundFile - info is not being updated automatically by Mate.
my idea now is to extract this info from the DCONF-editor
.
In Terminal
i tried without success:
gsettings get org.mate.background
gsettings get /org/mate/desktop/background/picture-filename
But In Terminal
the info of the BackgroundFile can be shown with:
dconf read /org/mate/desktop/background/picture-filename
Accordingly i tried to set the AccountsService
- user-files to:
[org.freedesktop.DisplayManager.AccountsService]
BackgroundFile=‘dconf read /org/mate/desktop/background/picture-filename’
[User]
Session=
XSession=mate
Icon=/home/standard/.face
SystemAccount=false
But this leads to show the Manjaro-standard login
and is not showing the users BackgroundFile.
What am i missing to ping this info into the user-files? maybe something like
{ ’ " [ $USER echo sudo gsettings dbus-launch ] " ’ } ???
i have no idea because i am a NOOB 