"You don't need to be an expert in Illustrator or Photoshop to follow this tutorial, as long as you have a good eye and a good amount of patience". - Breno
 
Step 1: Trace Your Photo. Start with a good photo that nice contrast. Shadow and highlight make it easy to see where to place your polygon shapes. I've chose to draw my shapes using Illustrator. (Smaller triangles for more details - eyes, nose and mouth).

Step 2: Coloring The Polygon Shape. Using the Direct Selection Tool (a) select a triangular mesh then grab the Eyedropper Tool (i) and choose any color inside the selected triangular mesh. Repeat this step until you filled in all mesh that you made.

Final Step: I adjusted the shapes and colors until i was completely satisfied, and finish it at Photoshop to apply final changes and accents.

Don't quit. Do it!