When is the background file copied to the VM, upon deployment via a scirpt or is it already baked in the template? If it's already in the template then you probably just need to use the Wow6432Node registry subkey. If the backgroud file is copied over during deployment then you'll also need to change the path to where the file is copied.
If the backgroud file is already in the template, follow this part:
Check this post:
http://www.turnon.dk/en/skift-logon-skaerm-pa-windows-7/
You need to edit the 32-bit section of the registry.
Instead of this:
"HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Authentication\LogonUI\Background" /v OEMBackground /f
Use this:
"HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Windows\CurrentVersion\Authentication\LogonUI\Background" /v OEMBackground /f
If the backgroud file is copied during deployment, then follow this to modify your script as the file is probably being copied to C:\Windows\SysWOW64\oobe\info\backgrounds instead of C:\Windows\system32\oobe\info\backgrounds.
Reference
http://msdn.microsoft.com/en-us/library/aa384187(v=vs.85).aspx
Instead of copying here, which will be redirected when accessed from a 32-bit app/script.
C:\Windows\system32\oobe\info\backgrounds
Copy here:
%windir%\Sysnative\oobe\info\backgrounds