@echo off
rem Just save as a bat file and set BackgroundImageYouWantToUse to the location of the image you want to use. 
REG /ADD HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Authentication\LogonUI\Background /t REG_SZ /v OEMBackground /d 1 /f
mkdir %windir%\system32\oobe\info
mkdir %windir%\system32\oobe\info\backgrounds

copy BackgroundImageYouWantToUse %windir%\system32\oobe\info\backgrounds\backgroundDefault.jpg