SOLVED: How to Change the Wallpaper on the Windows 8.1 Lock Screen Using Group Policy

If you want to force a background image to your Windows 8.1 Lock Screen you can accomplish your mission using a registry key or Group Policy that sets that registry key for you.

1. Download the Group Policy Templates for Windows 8.1 HERE and add them to your Group Policy Store (if you do not already have them)
2. Open your Group Policy Editor
   
3. Expand COMPUTER CONFIGURATION > POLICIES > ADMIN TEMPLATES > CONTROL PANEL > PERSONALIZATION
4. Double click FORCE A SPECIFIC DEFAULT LOCK SCREEN
5. Click ENABLED
6. Enter a path that is:
   1. local to all of your machines (i.e. you are going to copy the graphic to each of your machines)
   2. UNC to a public network share  (i.e. you could change this over time if you wished!)
7. Use a GPUPDATE /FORCE the reboot, or just wait a while for the GP to be applied to your PC then reboot