If you don't want to rename your stuff, the only thing you can do is to set it in the reg file that way:

[HKEY_CURRENT_USER\Control Panel\Desktop]

"Wallpaper"="C:\\WINDOWS\\Web\\Wallpaper\\WallpaperName.bmp"

Then copy your wallpaper (converted as a BMP, very important) to C:\Windows\Web\Wallaper

Good luck!

if you use a theme file you could create

$OEM$\$$\Resources\Themes

on your unattended install and copy your personal theme file in there. That way it's copied automatically.

Then you also need to modify winnt.sif

[shell]

CustomDefaultThemeFile = "%SYSTEMDRIVE%\Themes\MyTheme.theme"

.....maxXPsoft.... and then open in notepad XXXXX.theme file, find wallpaper's name and replace with your wallpaper name -> %windir%\Web\Walpaper\YYYYYY.jpg. Now you setup your default desktop Wallpaper.