#6 Visible Timer Countdown in Slideshow

I am using the slideshow to do drawing exercises: set the timer to 30 or 60 seconds and then quickly draw from photos of human beings in various poses. A similar exercise can be found at posemaniacs.com, but they use Poser models. Now, to be more useful, I would need a visible countdown clock to show me how much time I have left and not be surprised by the image change.