public delegate void CaptureWorkerDelegate(BitmapFrame frame, object state);
public static void CaptureScreenAsync(Uri source, TimeSpan timeSpan, object state, 
	CaptureWorkerDelegate finalWorkerPrimary)
{
	CaptureScreenAsync(source, timeSpan, -1, 
		state, finalWorkerPrimary, null);
}

public static void CaptureScreenAsync(Uri source, TimeSpan timeSpan, double scale, object state, 
	CaptureWorkerDelegate finalWorkerPrimary, CaptureWorkerDelegate finalWorkerThumbnail)
{
	ThreadPool.QueueUserWorkItem(
		delegate { CaptureScreen(source, timeSpan, scale, 
			state, finalWorkerPrimary, finalWorkerThumbnail); });
}

public static void CaptureScreen(Uri source, TimeSpan timeSpan, object state, 
	CaptureWorkerDelegate finalWorkerPrimary)
{
	CaptureScreen(source, timeSpan, -1, state, 
		finalWorkerPrimary, null);
}

public static void CaptureScreen(Uri source, TimeSpan timeSpan, double scale, object state, 
	CaptureWorkerDelegate finalWorkerPrimary, CaptureWorkerDelegate finalWorkerThumbnail)
{
	var player = new MediaPlayer { Volume = 0, ScrubbingEnabled = true };

	player.Open(source);
	player.Pause();
	player.Position = timeSpan;
	Thread.Sleep(1000);

	var width = player.NaturalVideoWidth;
	var height = player.NaturalVideoHeight;

	var rtb = new RenderTargetBitmap(width, height, 96, 96, PixelFormats.Pbgra32);
	var dv = new DrawingVisual();

	using (DrawingContext dc = dv.RenderOpen())
		dc.DrawVideo(player, new Rect(0, 0, width, height));

	rtb.Render(dv);
	var frame = BitmapFrame.Create(rtb).GetCurrentValueAsFrozen();
	if (finalWorkerPrimary != null)
		finalWorkerPrimary(frame as BitmapFrame, state);

	if (scale > 0 && finalWorkerThumbnail != null)
	{
		var thumbnailFrame = BitmapFrame.Create(
			new TransformedBitmap(frame as BitmapSource, 
				new ScaleTransform(scale, scale))).GetCurrentValueAsFrozen();
		var encoder = new JpegBitmapEncoder();
		encoder.Frames.Add(thumbnailFrame as BitmapFrame);

		finalWorkerThumbnail(thumbnailFrame as BitmapFrame, state);
	}
	
	player.Close();
}

private delegate void setImageDelegate(string controlName, BitmapFrame frame);
private void setImage(string controlName, BitmapFrame frame)
{
	var control = FindName(controlName);
	if (control != null)
		((Image)control).Source = frame;
}

private void makeThumbnails(BitmapFrame frame, object state)
{
	Dispatcher.Invoke(new setImageDelegate(setImage), (string)state, frame);
}

private void makeJpeg(BitmapFrame frame, object state)
{
	var encoder = new JpegBitmapEncoder();
	encoder.Frames.Add(frame);

	string filename = (string)state + ".jpg";
	using (var fs = new FileStream(filename, FileMode.Create))
		encoder.Save(fs);
}

private void Button_Click(object sender, RoutedEventArgs e)
{
	var source = video.Source;

	VideoScreenShot.CaptureScreenAsync(source, TimeSpan.FromSeconds(10), .1, 
		"image0", makeJpeg, makeThumbnails);
	VideoScreenShot.CaptureScreenAsync(source, TimeSpan.FromSeconds(43) + TimeSpan.FromMilliseconds(760), 
		"image1", makeThumbnails);
}