img := TIEBitmap.Create('tranparentMarker4.png');
  try
    if img.HasAlphaChannel then
    begin
      io := TImageEnIO.CreateFromBitmap(img.AlphaChannel);
      try
        io.SaveToFileBMP('alpha.bmp');
      finally
        io.Free;
      end;
    end;
  finally
    img.Free;
  end;

procedure TForm1.Button1Click(Sender: TObject);
var
  iIEBitmap: TIEBitmap;
  iImageENIO: TImageEnIO;
begin
  iIEBitmap := TIEBitmap.Create('TranparentMarker4.png');
  try
    if iIEBitmap.HasAlphaChannel then
    begin
       iImageENIO := TImageEnIO.CreateFromBitmap(iIEBitmap);
      try
        iImageENIO.Params.BMP_HandleTransparency := True;
        iImageENIO.SaveToFileBMP('alpha.bmp');
      finally
        iImageENIO.Free;
      end;
    end;
  finally
    iIEBitmap.Free;
  end;
end;