程序代码：

string imgUrl = "http://***/456.jpg";//目标图片
            WebRequest webReq = WebRequest.Create(imgUrl);
            WebResponse webRes = webReq.GetResponse();
            long fileLength = webRes.ContentLength;
            try
            {
                Stream srm = webRes.GetResponseStream();
                StreamReader srmReader = new StreamReader(srm);
                byte[] bufferbyte = new byte[fileLength];
                int allByte = (int)bufferbyte.Length;
                int startByte = 0;
                while (fileLength > 0)
                {
                    Application.DoEvents();
                    int downByte = srm.Read(bufferbyte, startByte, allByte);
                    if (downByte == 0) { break; };
                    startByte += downByte;
                    allByte -= downByte;
                }
                string tempPath = Application.StartupPath + "//download.jpg";//临时文件名
                //保存文件
                FileStream fs = new FileStream(tempPath, FileMode.OpenOrCreate, FileAccess.Write);
                fs.Write(bufferbyte, 0, bufferbyte.Length);
                srm.Close();
                srmReader.Close();
                fs.Close();
                //显示下载图片
                this.pictureBox1.Image = Image.FromFile(tempPath);
            }
            catch (WebException ex)
            {
                MessageBox.Show("下载失败！" + ex.Message.ToString(), "错误", MessageBoxButtons.OK, MessageBoxIcon.Error);
            }