uses Jpeg;

procedure TForm1.Button1Click(Sender: TObject);
var
  JpegIm: TJpegImage;
  bm: TBitMap;
begin
  bm := TBitMap.Create;
  bm.Width := Screen.Width;
  bm.Height := Screen.Height;
  BitBlt(bm.Canvas.Handle, 0, 0,
  bm.Width, bm.Height,
  GetDC(0), 0, 0, SRCCOPY);
  JpegIm := TJpegImage.Create;
  JpegIm.Assign(bm);
  JpegIm.CompressionQuality := 20;
  JpegIm.Compress;
  JpegIm.SaveToFile('C:\Screen.jpg');
  bm.Destroy;
  JpegIm.Destroy;
end;