procedure TForm1.Button1Click(Sender: TObject);
begin
  with Image1.Picture.Bitmap.Canvas do
  begin
    Font.Color  := clBlue;
    Brush.Style := bsclear;
    Font.Size   := 13;
    TextOut(10, 10, 'Transparent Text!!');
  end;
end;

{**** Or / Oder ****}

procedure TForm1.Button1Click(Sender: TObject);
var
  OldBkMode: integer;
begin
  with Image1.Picture.Bitmap do
  begin
    Canvas.Font.Color := clBlue;
    OldBkMode         := SetBkMode(Canvas.Handle, Ord(TRANSPARENT));
    Canvas.Font.Size  := 13;
    Canvas.TextOut(10, 10, 'Transparent Text!!');
    SetBkMode(Canvas.Handle, OldBkMode);
  end;
end;