异步FTP上载文件列表

我正在尝试循环浏览图像列表，并将它们ftp到服务器。它部分工作，除了它仍然阻塞我的UI线程。尽管我的ftp函数是异步的，但我猜因为我的调用方法不是，所以我没有得到我想要的结果。这是我得到的。我做错了什么？

   public void UploadPictures()
    {
        //loop through each picture and upload
        for (int i = 0; i < this.items.Count; i++) {
            byte[] bytes;
            if (System.IO.Path.GetExtension (this.items [i].FileName.ToUpper()) == ".JPG") {
                using (var imageData = this.items[i].Image.AsJPEG())
                {
                    bytes = new byte[imageData.Length];
                    Marshal.Copy(imageData.Bytes, bytes, 0, Convert.ToInt32(imageData.Length));
                }
                var test=UploadPhoto(bytes,  this.items[i].FileName);
            }
            if (System.IO.Path.GetExtension (this.items [i].FileName.ToUpper()) == ".PNG") {
                using (var imageData = this.items[i].Image.AsPNG())
                {
                    bytes = new byte[imageData.Length];
                    Marshal.Copy(imageData.Bytes, bytes, 0, Convert.ToInt32(imageData.Length));
                }
                var test=UploadPhoto(bytes,  this.items[i].FileName);
            }

        }

    }

    public static async Task<string> UploadPhoto(byte[] photoBytes,  string filename)
    {
        FtpWebRequest request = (FtpWebRequest)FtpWebRequest.Create("ftp://XXXXXXXX/" + filename);
        request.Method = WebRequestMethods.Ftp.UploadFile;
        request.Credentials = new NetworkCredential ("user", "pass"); 
        request.UseBinary = true;
        request.ContentLength = photoBytes.Length;
        using (Stream s = request.GetRequestStream())
        {
            s.Write(photoBytes, 0,photoBytes.Length);
        }
        WebResponse ftpResp = await (Task<WebResponse>)request.GetResponseAsync ();
        return ftpResp.ToString();
    }