Set the wallpaper(s) without changing options. Sets a different wallpaper on each monitor! :-)

Set-BingWallpaper.ps1

[CmdletBinding()]
param(
    # If you want to try the bing images from other countries, fiddle around with this (try en-GB, for instance)
    $Culture = 'en-US',
    # If you want to (re)use yesterday's wallpapers, fiddle around with this
    $Offset = 0
)

Add-Type -Assembly System.Windows.Forms, PresentationFramework
Add-Type -Name Windows -Namespace System -MemberDefinition @'
[DllImport("user32.dll", SetLastError = true, CharSet = CharSet.Auto)]
public static extern int SystemParametersInfo (int uAction, int uParam, string lpvParam, int fuWinIni);
'@


# create a bitmap the right size
$Height = [System.Windows.SystemParameters]::VirtualScreenHeight
$Width = [System.Windows.SystemParameters]::VirtualScreenWidth
# And make a note of the absolute position of the top left corder
$Top = [System.Windows.SystemParameters]::VirtualScreenTop
$Left = [System.Windows.SystemParameters]::VirtualScreenLeft

# Figure out how many wallpapers we need
$screens = [System.Windows.Forms.Screen]::AllScreens
$count = $screens.Count

# Fetch Bing's Image Archive information
# It would be fun to tell you about these images, right?
# TODO: We should do a notification with the details about the new wallpaper
$BingImages = Invoke-RestMethod "http://www.bing.com/HPImageArchive.aspx?format=js&idx=${Offset}&n=${count}&mkt=${Culture}"

$urlbase = "http://www.bing.com/"

$datespan = $Culture + $BingImages.images[-1].startdate + "-" + $BingImages.images[0].enddate

$TempPath = [System.IO.Path]::GetTempPath()

$WallPaperPath = Join-Path $TempPath "${datespan}.bmp"

if(!(Test-Path $WallPaperPath)) {
    $ErrorActionPreference = "Stop"
    Write-Verbose "Create Wallpaper from Bing images"
    $Wallpaper = New-Object System.Drawing.Bitmap ([int[]]($Width, $Height))
    $Graphics = [System.Drawing.Graphics]::FromImage($Wallpaper)

    for($i = 0; $i -lt $Count; $i++) {

        $Size = "{0}x{1}.jpg" -f $Screens[$i].Bounds.Width, $Screens[$i].Bounds.Height
        $File = Join-Path $TempPath ($BingImages.Images[$i].startdate + "_" + $Culture + "_" + $Size)

        # Download the image (assume they'll make one at my resolution)
        if(!(Test-Path $File)) {
            $ImageUrl = $urlbase + $BingImages.Images[$i].UrlBase + "_" + $Size
            Write-Verbose "Download Image $ImageUrl to $File"
            Invoke-WebRequest $ImageUrl -OutFile $File
        } else {
            Write-Verbose "Using cached image file $File"
        }

        Write-Verbose ("Draw $File to {0}, {1}" -f ($Screens[$i].Bounds.X - $Left), ($Screens[$i].Bounds.Y - $Top))
        $Source = [System.Drawing.Image]::FromFile($File)

        # Putting the wallpaper in the right place, relatively speaking, is the tricky bit
        $Graphics.DrawImage($Source, $Screens[$i].Bounds.X - $Left, $Screens[$i].Bounds.Y - $Top)
    }

    $Graphics.Dispose()

    $Wallpaper.Save($WallPaperPath)
} else {
    Write-Verbose "Update wallpaper from cached image file $WallPaperPath"
}

[Windows]::SystemParametersInfo( 20, 0, $WallPaperPath, 0x03)