Once you have tagged devices and added them to groups, open your dashboard https://commbox.glbth.com/index.html#/dashboard, on the side menu click groups, choose the group you wish to manage and click the 3 dots to open the actions menu. Open settings and select 'Add New'.
Select wallpaper from the left side and turn the 'Set Device Wallpaper' toggle on so it is no longer grey.
Select the blue icon next to 'Upload File' and upload any 1920x1080p jpg file and hit Confirm.
Select the new wallpaper setting you have created and press apply. This action is almost instantaneous but all devices need to be connected to the network for this to work.