I've done a few photorealistic sketches in my time. I would say the best so far is one I did of Philip Seymour Hoffman many years ago. Each of these were drawn by hand from a reference photograph using pencils.
I wanted to give digital artwork a try, so I created this skating penguin picture. The line art is hand drawn. Once that was completed, I created a digital copy of the line art and created a vectorized version of it. From there, I moved on to creating flats and filling in the final colors using a graphics tablet on my computer and Adobe Photoshop. Below, you will find the final result, along with snapshots of the progess throughout the creation process. I plan on doing more of these in the future as I find the time to do them.