Boxstarter

set-itemproperty -path "HKCU:Control Panel\Desktop" -name WallPaper -value 1.jpg
rundll32.exe user32.dll, UpdatePerUserSystemParameters

Function Set-WallPaper($Value)
{
 Set-ItemProperty -path 'HKCU:\Control Panel\Desktop\' -name wallpaper -value $value
 rundll32.exe user32.dll, UpdatePerUserSystemParameters
}