在后台添加图片（使用iTextSharp）

Question

我尝试在pdf文档的所有页面中添加徽标。 我的代码：

private string insertBgImage(string doc)
    {
        PdfReader reader = new PdfReader(doc);
        int NumOfPages = reader.NumberOfPages;

        //MemoryStream memoryStream = new MemoryStream();
        //memoryStream.ToArray();
        string newDoc = Path.Combine(Path.GetTempPath(), "resume1.pdf");
        FileStream fileStream = new FileStream(newDoc, FileMode.Create);
        PdfStamper stamp = new PdfStamper(reader, fileStream);
        int i = 1;
        PdfContentByte under;
        PdfContentByte over;

        string bgPath = SPUtility.GetGenericSetupPath(@"Template\Images\ResumeList");
        string BgImgUri = Path.Combine(bgPath, "bg.gif");
        iTs.Image bg = iTs.Image.GetInstance(new Uri(BgImgUri));
        BaseFont bf = BaseFont.CreateFont(BaseFont.HELVETICA, BaseFont.WINANSI, BaseFont.EMBEDDED);
        bg.ScaleToFit(300, 337);
        float absX = (PageWidth - 300) / 2;
        float absY = (PageHeight - 337) / 2;
        bg.SetAbsolutePosition(absX, absY);
        bg.Alignment = iTs.Image.UNDERLYING;

        while (i <= NumOfPages)
        {
            under = stamp.GetUnderContent(i);
            under.AddImage(bg);

            //over = stamp.GetOverContent(i);
            //over.AddImage(bg);
            //over.BeginText();
            //over.SetFontAndSize(bf, 10);
            //over.ShowText("page" + i);
            //over.EndText();

            i++;
        }

        stamp.Close();

        return newDoc;
    }

但是这个： under.AddImage(bg); 不工作