This script adds registry entries pointing to a stored bmp on the machines hard drive, it changes the logon screen background and the desktop background, hope this is useful to you:
Set WshShell = WScript.CreateObject("WScript.Shell")
WshShell.RegWrite"HKEY_USERS\.DEFAULT\Control Panel\Desktop\Wallpaper","C:\test.bmp","REG_SZ"
WshShell.RegWrite"HKEY_USERS\.DEFAULT\Control Panel\Desktop\WallpaperStyle","0","REG_SZ"
WshShell.RegWrite"HKEY_CURRENT_USER\Control Panel\Desktop\Wallpaper","C:\test.bmp","REG_SZ"
WshShell.RegWrite"HKEY_CURRENT_USER\Control Panel\Desktop\WallpaperStyle","0","REG_SZ"
wscript.echo "Wallpaper settings have been changed"