procedure TForm1.Button1Click(Sender: TObject);

var

  bmp: TBItmap;

  jpg: TJpegImage;

  scale: Double;

begin

  if opendialog1.execute then

  begin

    jpg := TJpegImage.Create;

    try

      jpg.Loadfromfile( opendialog1.filename );

      if jpg.Height > jpg.Width then

        scale := 50 / jpg.Height

      else

        scale := 50 / jpg.Width;

      bmp:= Tbitmap.Create;

      try

        {Create thumbnail bitmap, keep pictures aspect ratio}

        bmp.Width := Round( jpg.Width * scale );

        bmp.Height:= Round( jpg.Height * scale );

        bmp.Canvas.StretchDraw( bmp.Canvas.Cliprect, jpg );

        {Draw thumbnail as control}

        Self.Canvas.Draw( 100, 10, bmp );

        {Convert back to JPEG and save to file}

        jpg.Assign( bmp );

        jpg.SaveToFile(ChangeFileext( opendialog1.filename, '_thumb.JPG' ));

      finally

        bmp.free;

      end;

    finally

      jpg.free;

    end;

  end;

end;