- Start the group policy editor (gpedit.msc).
- Go to Computer Configuration / Administrative Templates / System / Logon
- Enable "Always use custom logon background".
- Create the directory C:\Windows\System32\oobe\info\backgrounds if it doesn't already exist.
- Put the desired login wallpaper in that directory, with the name backgroundDefault.jpg. This will need to be 256kb or less.
Interestingly, after fiddling around with various wallpapers, I have ended up with the Linux login wallpaper being a dark blue version of the Windows desktop wallpaper, and vice-versa:
- Put the desired image file somewhere where the LightDM greeter (the program which handles logging in and out) can see it. Because your home directory may not be accessible to the greeter before you log in (mine isn't, which caused a problem with the Simple LightDM Manager program which insists on copying wallpaper files to your home directory), this should ideally be somewhere like /usr/share/backgrounds.
- Edit /etc/lightdm/unity-greeter.conf to include the line
The orange Linux wallpaper is a macro photo of a nectarine, and the light blue Windows wallpaper is the water rushing by the Poole harbour ferry.