如何比较图像(缓存图像)

我正在metro风格的应用程序中进行简单的图像缓存。以下是我已经做过的：

    private async void GetImage()
    {
        bool isFolderExisting = true;
        bool isFileExisting = true;
        StorageFolder storageFolder = null;
        StorageFile storageFile = null;
        // get screenshots folder    
        try
        {
            storageFolder = await ApplicationData.Current.TemporaryFolder.GetFolderAsync("screenshots");
        }
        catch (System.IO.FileNotFoundException ex)
        {
            isFolderExisting = false;
        }
        // if folder doesn't exist, create new one
        if (isFolderExisting == false)
            storageFolder = await ApplicationData.Current.TemporaryFolder.CreateFolderAsync("screenshots");
        // get screenshot
        try
        {
            storageFile = await storageFolder.GetFileAsync(this.LinkId);
            //IAsyncAction threadPoolWorkItem = ThreadPool.RunAsync((source) => { updateImage(storageFile); });
        }
        catch (System.IO.FileNotFoundException ex)
        {
            isFileExisting = false;
        }
        // if file doesn't exists, download and save new one
        if (isFileExisting == false)
        {
            var uri = new Uri(WebServiceAddress + "/screenshot/" + this.LinkId, UriKind.Absolute);
            var thumbnail = Windows.Storage.Streams.RandomAccessStreamReference.CreateFromUri(uri);
            storageFile = await StorageFile.CreateStreamedFileFromUriAsync(this.LinkId, uri, thumbnail);
            await storageFile.CopyAsync(storageFolder, storageFile.Name, NameCollisionOption.ReplaceExisting);
        }
        //this.ImageSource = new Windows.UI.Xaml.Media.Imaging.BitmapImage(new Uri("ms-appdata:///temp/screenshots/" + this.LinkId)); 
        this.Image = "ms-appdata:///temp/screenshots/" + this.LinkId;
    }

现在我要处理最后一部分，那就是比较图像。我正在检查临时文件夹中是否存在图像。如果它不存在，我只是下载新的，但如果存在，我需要检查它是否与服务器上的相同。我怎样才能做到这一点？