// Based on 6 Aug 2000 post to borland.public.delphi.objectpascal
// by Peter Klein.
procedure TForm1.ButtonJPGClick(Sender: TObject);
  CONST
    BufferSize = 50;
  VAR
    Buffer     :  STRING;
    index      :  INTEGER;
    FileStream :  TFileStream;
    xResolution:  WORD;
    yResolution:  WORD;
begin
  IF   OpenDialog.Execute
  THEN BEGIN
    FileStream := TFileStream.Create(OpenDialog.FileName,
                                     fmOpenRead OR fmShareDenyNone);
    TRY
      SetLength(Buffer, BufferSize);
      FileStream.Read(buffer[1], BufferSize);
      index := POS('JFIF'+#$00, buffer);
      IF   index > 0
      THEN BEGIN
        FileStream.Seek(index+7, soFromBeginning);
        FileStream.Read(xResolution, 2);
        FileStream.Read(yResolution, 2);

        xResolution :=  Swap(xResolution);
        yResolution :=  Swap(yResolution);

        ShowMessage('xResolution=' +
                    IntToStr(xResolution) +
                    ', yResolution=' +
                    IntToStr(yResolution))
      END
    FINALLY
      FileStream.Free
    END
  END
end;