app 调用webservice的图片上传接口报错

以下为c#写的图片上传接口，图片是在app转为base64的；而当在服务上测试时是可以上传，即下图的位置
可是在app调用该接口时却出错，这是为什么，困扰了多天了。请大神帮忙哈

public JObject UploadPicture(string jsonString)
{
JToken Picture;
JObject jobject = JObject.Parse(jsonString);
Picture = jobject.GetValue("Picture");
JObject jObj = new JObject();
int re = 0;
string imgStr = Picture.ToString(),pictureAddress;
string picture = DateTime.Now.Ticks.ToString() + ".jpg", filePath;
filePath = "~/Resources/images/" + picture;

//将base64图片解析出来
if (imgStr == "")
{
  re = 0;
}
else
{
  byte[] LogoBytes = Convert.FromBase64String(imgStr);//base64转二进制
  MemoryStream stream = new MemoryStream(LogoBytes);//二进制转流
  Image img1 = Image.FromStream(stream);//流转image
  if (Directory.Exists(Server.MapPath("~/Resources/images")) == false)
  {
    Directory.CreateDirectory(Server.MapPath("~/Resources/images"));
  }
  string add = Server.MapPath(filePath);//地址
  Bitmap server = new Bitmap(img1);
  try
  {
    server.Save(add);
    re = 1;
  }
  catch
  {
    re = 0;
  }
  stream.Dispose();
  server.Dispose();
  img1.Dispose();
}
jObj.Add(new JProperty("re", re));
if (re == 1)
{
  pictureAddress = "/wd_test/Resources/images/" + picture;
  jObj.Add(new JProperty("pictureAddress", pictureAddress));
}

return jObj;

}