In our images staff wanted a custom OEM background with the image version in the lower right corner and both the wallpaper and user icon branded to a TMCC default picture but not forced via GPO so users could change it if they want to.

Creating a custom logon was the easy part.  Set the registry key [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Authentication\LogonUI\Background] "OEMBackground"=dword:00000001  and place a files named backgrounddefault.jpg (must be less then 256k) in a folder C:\Windows\System32\oobe\info\backgrounds.  logoff and it should be there. 

Now changing the default wallpaper and user icon was a problem. I replaced the file C:\Windows\Web\Wallpaper\Windows\img0.jpg for the wallpaper and the C:\ProgramData\Microsoft\User Account Pictures\user.bmp Windows detected the pictures where not the orginal put them back to the default version when I rebooted.  My quick solution was to edit the two files and paste in what I wanted and then save them.  Had to take ownership of the files to do this I then deleted C:\Users\default\AppData\Roaming\Microsoft\Windows\Themes\transcodedwallpaper.jpg all new users then got the changes. 

A sysprep image is much easier I copy a user profile over default so just had to change files and choose the background and then cancel and it changed to the new one.