Thanks Richard,
There doesn't seem to be anywhere in GP to set a desktop wallpaper apart from if you are using active desktop.
I have tried this in the Kix (4.50), it changes the background colour but does not add the logo.bmp. It is 197kb in size if that makes a difference.

$Result = WriteValue("HKEY_CURRENT_USER\Control Panel\Desktop","TileWallpaper","1568","REG_SZ")
$Result = WriteValue("HKEY_CURRENT_USER\Control Panel\Colors","Background","0 0 0","REG_SZ")
if Exist("$Netlogon\logo.bmp")
$Result = SetWallpaper("$Netlogon\logo.bmp")
else
$Result = SetWallpaper("")
endif