asp.net MVC怎么给图input type=file传过来的图片加水印

加水印后，在controller里面传到服务器里面保存

这是controller的代码
public ActionResult AddNewsPic(string id)
{

        if (Request.Files.Count > 0)
        {
            if (!string.IsNullOrEmpty(Request.Files[0].FileName))
            {
                Stream fileDataStream = null;
                int filelength = 0;
                byte[] byteImgData = null;
                fileDataStream = Request.Files[0].InputStream;
                filelength = Request.Files[0].ContentLength;
                byteImgData = new byte[filelength];//存储二进制
                fileDataStream.Read(byteImgData, 0, filelength);

                string eid = Request.Form["id"].ToString();
                string Summary = Request.Form["Summary"].ToString();
                NewsBLL poluBLL = new NewsBLL();
                bool temp = poluBLL.AddNewsPic(null, Summary, byteImgData, eid);


            }

        }
        ViewBag.eID = id;
        return View();
    }

怎么在controller中图片上传之前，对图片加一个文字水印