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;