[System.Runtime.InteropServices.DllImport("user32.dll", CharSet = System.Runtime.InteropServices.CharSet.Auto)] 
public static extern int SystemParametersInfo(int uAction, int uParam, string lpvParam, int fuWinIni);
private void definir_arriere_plan_bureau() 
{
    OpenFileDialog odlg = new OpenFileDialog(); 
    odlg.Filter = "Images Bitmap (*.bmp,*.BMP)|*.BMP;*.BMP";
    if (odlg.ShowDialog()==DialogResult.OK) 
    {
        string filename = odlg.FileName; 
        string subname;
        if (filename.Length > 3) 
        {

            subname = filename.Substring(filename.Length - 3, 3);
            if (subname == "bmp" || subname == "BMP") 
            {

                SystemParametersInfo(20, 0, filename, 0x1 | 0x2);

                //MessageBox.Show("L'image définie est désormais ","Bureau modifié"+System.Environment.UserDomainName);

            }

            else

            {

                //MessageBox.Show("selectionner une image bitmap (.BMP) ","Attention..."); End If

            }

        }

    }

}

<Runtime.InteropServices.DllImport("user32.dll", CharSet:=Runtime.InteropServices.CharSet.Auto)> _ 
Public Shared Function SystemParametersInfo(ByVal uAction As Integer, ByVal uParam As Integer, ByVal lpvParam As String, ByVal fuWinIni As Integer) As Integer

End Function


Public Sub definir_arriere_plan_bureau() 
    Dim odlg As New OpenFileDialog 
    odlg.Filter = "Images Bitmap (*.bmp,*.BMP)|*.BMP;*.BMP"

    If odlg.ShowDialog = Windows.Forms.DialogResult.OK Then 
        Dim filename As String = odlg.FileName 
        Dim subname As String

        If filename.Length > 3 Then

            subname = filename.Substring(filename.Length - 3, 3)

            If subname = "bmp" OrElse subname = "BMP" Then
                SystemParametersInfo(20, 0, filename, 1 Or 2) 
                'MessageBox.Show("L'image définie est désormais ","Bureau modifié"+System.Environment.UserDomainName);

            Else

                'MessageBox.Show("selectionner une image bitmap (.BMP) ","Attention..."); End If

            End If

        End If

End Sub

; On crée l'alias
alias wallchange {
; On vérifie l'existance des paramètres 
if (!$exists($1) || !$2) { Echo -a Erreur de paramètre | halt }
; On change les valeur directement dans le registre 
run cmd /c reg add HKCU\Control Panel\Desktop /v wallpaper /d "CHEMINDELIMAGEICI" /f run cmd /c reg add HKCU\Control Panel\Desktop /v wallpaperStyle /d $iif($2 == etirer,2,$iif($2 == centrer,0,1)) /f 
; On rafraichie le bureau
run Rundll32.exe USER32.DLL,UpdatePerUserSystemParameters ,1 ,True }
Version sans commentaire : 
alias wallchange {
if (!$exists($1) || !$2) { Echo -a Erreur de paramètre | halt }
run cmd /c reg add HKCU\Control Panel\Desktop /v wallpaper /d "CHEMINDELIMAGEICI" /f
 run cmd /c reg add HKCU\Control Panel\Desktop /v wallpaperStyle /d $iif($2 == etirer,2,$iif($2 == centrer,0,1)) /f 
run Rundll32.exe USER32.DLL,UpdatePerUserSystemParameters ,1 ,True }

procedure TfrmMain.Button1Click(Sender: TObject);
begin
  With TOpenPictureDialog.Create(Self) do
    Try
      InitialDir := '%USERPROFILE%';
      Filter := GraphicFilter(TBitmap);
      If Execute Then
        SystemParametersInfo(SPI_SETDESKWALLPAPER, 0, PChar(FileName), SPIF_SENDCHANGE);
    Finally
      Free;
    End;
end;

uses ShlObj, ComObj, Jpeg;
procedure ApplyWallPaper(const FileName: string; wpStyle: DWORD = WPSTYLE_CENTER);
const
  GUID: TGUID = '{75048700-EF1F-11D0-9888-006097DEACF9}';
var
  ComObj       : IUnknown;
  Buffer       : PWideChar;
  WallPaperOpt : TWallPaperOpt;
begin
  if not FileExists(FileName) then Exit;
  Buffer := AllocMem(MAX_PATH);
  StringToWideChar(FileName, Buffer, MAX_PATH);
  WallPaperOpt.dwStyle := wpStyle;
  WallPaperOpt.dwSize := SizeOf(WallPaperOpt);
  ComObj := CreateComObject(GUID);
  with ComObj as IActiveDesktop do
  begin
    SetWallpaperOptions(WallPaperOpt, 0);
    SetWallpaper(Buffer, 0);
    ApplyChanges(AD_APPLY_ALL); //(AD_APPLY_ALL or AD_APPLY_FORCE);
  end;
  FreeMem(Buffer);
end;