Hello @Panagiotis Chatzilabrou,
Thank you for posting in our Q&A forum.
Based on the description"So I activated and changed all the wallpapers with GPO", would you please
tell us how you deploy all the wallpapers with GPO?
If you deploy wallpaper using the GPO setting under "User Configuration > Administrative Templates > Desktop > Desktop > “Desktop Wallpaper”, whether you put wallpaper file in a local path or a network path?
If it is a network path and wallpaper will be black if there is no network.
Also, you can deploy wallpapers through the steps in the following similar thread.
https://social.technet.microsoft.com/Forums/en-US/a38dac9d-edd0-41ef-a3f7-b6be3bd0bc12/how-to-deploy-desktop-wallpaper-through-group-policy-for-offline-user-in-server-2012
Hope the information above is helpful. If you have any question or concern, please feel free to let us know.
Best Regards,
Daisy Zhou
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