封存Image控件source 至本地出错

保存Image控件source 至本地出错
我在保存Image控件的图片时，老是出错，代码如下，请大家看看，谢谢

string message = "保存成功";
            try
            {
                BitmapSource source = ImagePanel.Source as BitmapSource;
                if (source == null)
                    return;
                WriteableBitmap wb = new WriteableBitmap(source);
              
                using (MemoryStream stream = new MemoryStream(50000))
                {
                    wb.SaveJpeg(stream, wb.PixelWidth, wb.PixelHeight, 0, 100);
                    stream.Seek(0, SeekOrigin.Begin);
                    Guid guid = Guid.NewGuid();
                    try
                    {
                        PhoneUtils.SavePicture(stream, guid.ToString() + ".jpg");
                    }
                    catch (Exception ex)
                    {
                        message = "保存失败,错误：" + ex.Message;
                    }
                }
            }
            catch (Exception ex)
            {
                message = ex.Message;
            }