/// <summary>
        /// 获取图片路径
        /// </summary>
        /// <param name="httpUrl"></param>
        /// <returns></returns>
        public string GetImageUrl(string httpUrl)
        {
            HttpWebRequest req = (HttpWebRequest)WebRequest.Create(httpUrl);
            req.ServicePoint.Expect100Continue = false;
            req.Method = "GET";
            req.KeepAlive = true;
            req.ContentType = "image/png";
            HttpWebResponse rsp = (HttpWebResponse)req.GetResponse();
            StreamReader reader = new StreamReader(rsp.GetResponseStream(), Encoding.GetEncoding("utf-8"));
            string str = reader.ReadToEnd();
            string[] imgStr = GetHtmlImageUrlList(str);
            string strer = "https://open.weixin.qq.com" + imgStr[0];
            return strer;
        }
        /// <summary>
        /// 获取img标签
        /// </summary>
        /// <param name="sHtmlText"></param>
        /// <returns></returns>
        public string[] GetHtmlImageUrlList(string sHtmlText)
        {
            Regex regImg = new Regex(@"<img\b[^<>]*?\bsrc[\s\t\r\n]*=[\s\t\r\n]*[""‘]?[\s\t\r\n]*(?<imgUrl>[^\s\t\r\n""‘<>]*)[^<>]*?/?[\s\t\r\n]*>", RegexOptions.IgnoreCase);
            MatchCollection matches = regImg.Matches(sHtmlText);
            int i = 0;
            string[] sUrlList = new string[matches.Count];
            foreach (Match match in matches)
                sUrlList[i++] = match.Groups["imgUrl"].Value;
            return sUrlList;
        }

string imgUrl= GetImageUrl("https://open.weixin.qq.com/......微信地址");
            img.Source = new BitmapImage(new Uri(imgUrl));