iTextSharp add Image to Old PDF

iTextSharp对PDF文件修改功能是相当的弱智，呵呵。
private void button3_Click(object sender, System.EventArgs e)
  {
   PdfReader reader = new PdfReader(@"4.pdf");

   int n = reader.NumberOfPages;   

   PdfStamper stamp = new PdfStamper(reader, new FileStream("Test1Watermark.pdf",FileMode.Create));
  
   int i = 0;
   PdfContentByte under;
   iTextSharp.text.Image img = Image.GetInstance("jpg.jpg");
   img.SetAbsolutePosition(516,823);
   img.ScaleAbsolute(100,100);

   Image mm = Image.GetInstance("mm.jpg");
   mm.SetAbsolutePosition(500,810);
   mm.ScaleAbsolute(300,100);
   
   while (i < n)
   {
    i++;
    //under = stamp.GetUnderContent(i);
    under=stamp.GetOverContent(i);
    under.
    //under.RoundRectangle(100,100,100,100,100);
    PdfTemplate template = under.CreateTemplate(300,300);
    under.AddImage(img);
    under.AddImage(mm);
    
    template.AddImage(img);
    under.AddTemplate(template,200,400);
   }

   stamp.Close();
   reader.Close();
   

   
  }