TDbCtrlGrid: how to draw a transparent image?

Hi,
I've a TDBGrid-component and want to draw images on it depending from an integer-value in the database. It'works well, but I can't make the images transparent. There are simple images, I put the images-background to color clBtnFace and draw i.e a green rectangle. On the CtrlGrid the background of the image is always white. I tried to set the "TransparentColor" but without success. The images are 24Bit. Her's my code:

procedure TForm1.DBCtrlGrid1PaintPanel(DBCtrlGrid: TDBCtrlGrid;
  Index: Integer);
var
  bitmap : TBitmap;
  fixRect : TRect;
  vTyp : integer;
begin
    vTyp :=  DBCtrlGrid.DataSource.DataSet.FieldByName('Typ').AsInteger;
    if vTyp < ImageList1.Count then
    begin
      bitmap := TBitmap.Create;
      try
        ImageList1.GetBitmap(vTyp,bitmap);
        fixRect.Top := 0;
        fixRect.Bottom := fixRect.Top+32;
        fixRect.Left := E_Typ.Left+ E_Typ.Width + 3;
        fixRect.Right := fixRect.Left+32;
        DBCtrlGrid.Canvas.StretchDraw(fixRect,bitmap);
      finally
        FreeAndNil(bitmap);  
      end;
    end;
end;

Thanks for your help, Peter