How can I compare images (caching images)?
===

I'm doing simple image caching in my metro style application. Here is what I've already done:

    	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;
        }

Now I have to take care of the last part which is comparing images.
I'm checking if image exists in temp folder. If it doesn't exists I'm just downloading new, but if exists I need to check if it's the same as on server. How can I achive that?