try {
            PdfReader Read_PDF_To_Watermark = new PdfReader("Sample.pdf");
            int number_of_pages = Read_PDF_To_Watermark.getNumberOfPages();
            PdfStamper stamp = new PdfStamper(Read_PDF_To_Watermark, new FileOutputStream("New_PDF_With_Watermark_Image.pdf"));
            int i = 0;
            Image watermark_image = Image.getInstance("watermark.jpg");
            watermark_image.setAbsolutePosition(200, 400);
            PdfContentByte add_watermark;            
            while (i < number_of_pages) {
              i++;
              add_watermark = stamp.getUnderContent(i);
              add_watermark.addImage(watermark_image);
            }
            stamp.close();
        }
        catch (Exception i1) {
            i1.printStackTrace();
        }

------解决方案--------------------

Image watermarkImage = new Image(imgPath);
watermarkImage.setAbsolutePosition(x, y);

writer.setPageEvent( new MyPageEvent(watermarkImage) );


public MyPageEvent extends PdfPageEventHelper {
 private Image waterMark;
 public MyPageEvent(Image img) {
   waterMark = img;
 }
 public void OnEndPage(PdfWriter writer, Document doc) {
   PdfContentByte content = writer.getContent();
   content.addImage( waterMark );
 }
}