Logon Screen Background
The Logon Screen Background feature lets you customize the picture that the system displays on background of the Logon screen and/or the Lock screen. On multi-monitor systems, it also allows to run picture slideshow on secondary monitors (by default, the system shows nothing there). Due to peculiarity of system internals, this feature works differently on different versions of Windows and on primary/secondary displays.
On Windows Vista and Windows 7, background picture is the same for both the Logon screen and the Lock screen. Also, it's the same for all users.
Since Windows 8, there is no ability to change a picture displayed on the Logon screen because the system does not allow to customize it. But the system allows to customize a picture displayed on the Lock screen, and each user can have its own Lock screen background picture.
You can customize the primary monitor background on the Logon Screen - Primary monitor panel.
Displaying background pictures on secondary displays works the same on all systems since Windows Vista (on older systems this feature is unavailable). Also, it uses the same settings for all users.
Be careful when adding picture file folders as slideshow sources on a system with more than one user account: DO NOT add any folder with private data if you do not want other users of the system see your private pictures.
Unlike the primary monitor, you can provide more than a single picture for each secondary monitor, i.e. have custom slideshow on each of your monitors.
If you nevertheless want to have a static picture on a certain secondary monitor then just add a single picture file into that monitor's slideshow playlist.
You can customize secondary monitors' slideshows on the Logon Screen - Secondary monitors panel.