为什么调用保存图片到手机的SavePicture方法会抛出错误

为什么调用保存图片到手机的SavePicture方法会抛出异常
MediaLibrary library = new MediaLibrary();
            string lName = "picture1.jpg";
            var myStore = IsolatedStorageFile.GetUserStoreForApplication();
            if (myStore.FileExists(lName))
            {
                MessageBox.Show("图片已经存在");
            }
            else
            {
                IsolatedStorageFileStream myFileStream = myStore.CreateFile(lName);
                WriteableBitmap _bitmap = new WriteableBitmap(inkPresenter, null);
                Extensions.SaveJpeg(_bitmap, myFileStream, _bitmap.PixelWidth, _bitmap.PixelHeight, 0, 85);
                myFileStream.Close();
                myFileStream = myStore.OpenFile(lName, FileMode.Open, FileAccess.Read);
                try
                {
                    Picture pic = library.SavePicture(lName, myFileStream);
                    MessageBox.Show("保存成功！", "提示", MessageBoxButton.OK);
                }
                catch (Exception)
                {
                    MessageBox.Show("异常");
                }
                myFileStream.Close();