I'm having problems getting this to work.
I'm using 64-bit WIndows 7. I've done the changes outlined with adding a registry entry and a defaultBackground.jpg, but that doesn't work. Apparently for 32-bit Windows. Don't know.
I've done replacing background.bmp the file in syswow64\oobe folder. That doesn't work.
The little tools I can download all show the new background, but Windows doesn't actually use it. I've rebooted many, many times.