Hello @rkelegha I have both a custom background or desktop picture along with a custom login window.
The desktop picture, I loaded the possible desktop photos, I have many, into the /Library/Desktop Pictures folder. Then I use a configuration profile that has a restriction payload. I put in the restriction the path to that desktop photo and scoped accordingly. Make sure to grant permissions so the /Library/Desktop Pictures folder and its contents have the same permission as the desktop pictures folder, I did this using the get Info window. Then all the scoped computers that have the desktop photo loaded in their desktop pictures folder will see it and only be able to use it.
As for the login window, I simply created a photo or image called it com.apple.desktop.admin.png. I copied this image to the /Library/Caches folder. Make sure to grant permissions so the folder and its contents are the same as the /Library/Caches folder permission. Again, I did this using the get info window. I then captured this file in composer by dragging the com.apple.desktop.admin.png file into the composer window and made a dmg file that I can send to all the computers using a policy of choice. All the computers at the login window will have this new image.
Hopefully this helps.
