Change the OEMBackground registry key from 0 to 1. The location is outlined below:

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Authentication\LogonUI\Background]
“OEMBackground”=dword:00000001

Navigate to:

C:\Windows\System32\oobe\info\backgrounds

Rename the existing image from backgroundDefault.jpg to something else like backgroundDefault.jpg.orig.

Then, you are going to put the image you want in this directory. The key is making the image you replace it with is 256Kb or less, and is a jpg. Once you have such an image that you want to use, put it in the “C:\Windows\System32\oobe\info\backgrounds” directory and name it backgroundDefault.jpg.