Delphi XE8 Maps Marker Generation with transparent areas

Question

I'm currently working on a logistics application. With Delphi XE8 and TMS Maps. Works great, but I have a problem with markers generated in the program.

The difficulty I'm having is that the markers are generated as desired except that I have a clFuchsia rectangle around each marker. See the picture below. Maybe someone can give me a hint how to make this Fuchsia area transparent. Thanks in advance HeWy

And here my Marker generator function:

Function tMarkerUtils.GenerateTriMarker(aColor: tColor): string;

var MarkerImage: TImage;
    TransparentColor: TColor;
    Points: array of TPoint;

begin
  MarkerImage:= TImage.Create(nil);
  try
    MarkerImage.Width:= 55;
    MarkerImage.Height:= 30;
    MarkerImage.Picture:=nil;

    TransparentColor := clFuchsia; // Background color

    MarkerImage.Canvas.Brush.Style := bsClear;
    MarkerImage.Canvas.Brush.Color:= TransparentColor;
    MarkerImage.Picture.Bitmap.TransparentColor := TransparentColor;
    MarkerImage.Picture.Bitmap.Transparent:= true;
    MarkerImage.Canvas.FillRect(Rect(0, 0, MarkerImage.Width, MarkerImage.Height));

    SetLength(Points, 3);
    Points[0] := Point(5, 5);
    Points[1] := Point(35, 5);
    Points[2] := Point(20, 20);
    MarkerImage.Canvas.Pen.Width := 2;
    MarkerImage.Canvas.Pen.Color := clBlack;
    MarkerImage.Canvas.Brush.Color := aColor;
    MarkerImage.Canvas.Polygon(Points);

    Result:= GetBase64Image(MarkerImage);
  finally
    MarkerImage.Free;
  end;
end;