Я создал функцию по рекомендации Christian.K
function Get-LockscreenWallpaperPath () {
$null = [Windows.System.UserProfile.LockScreen,Windows.System.UserProfile,ContentType=WindowsRuntime]
$Path = [Windows.System.UserProfile.LockScreen]::OriginalImageFile.AbsolutePath
$Path = $Path -replace '/','\'
return $Path
}