How to Change Windows Logon Background Image Manually

You can manually change that by following some steps and tweaking your registry entry, so check below steps on how you can do that.

1. Go to Start > Run and type regedit to open the Registry Editor.
2. Now Navigate to HKEY_LOCAL_MACHINE > SOFTWARE > Microsoft > Windows > CurrentVersion > Authentica​tion > LogonUI > Background. 
3. Now you will see OEMBackground value if its not there create one by right clicking and choosing New > DWORD Value, and rename it too OEMBackground.
4. Now just double click on the OEMBackground value and replace 0 with 1.
5. Now close the registry and navigate to C: > Windows > System32 > oobe.
6. Now just create a new folder names "info" and create another sub folder in it and name it as "backgrounds"
7. Now just add your desired wallpaper in this folder and rename that image as backgrounddefault.jpg, make sure that images must be less than 245KB in size.
8. That’s it now just Log Off your system and Login again.