Step 1: Open the Windows Start menu and enter regedit into the search bar.
Step 2: Right click on HKEYLOCALMACHINE and select Find.
Step 3: Search for OEMBackground, which should be in Computer\HKEYLOCALMACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Authentication\LogonUI\Background. If the key does not exist, add a new DWORD value with the name OEMBackground.
Step 4: Double click on the entry OEMBackground, and change the value from 0 to 1. Step 5: Open Windows Explorer and go to the following path: %windir%\system32\oobe.
Step 6: Create a new folder named info, and open it.
Step 7: Create a new folder within info named backgrounds.
Step 8: Rename your desired wallpaper as backgroundDefault.jpg and place it inside the folder backgrounds (images must be less than 245KB in size).
Now reboot and you should be good :)