I get a lot of questions about sketching from photos. I thought I'd share some observations on the idea.
The first thing I'll share with you is that size matters, the bigger the image the better. It's much easier to make something smaller in size than to make it larger (enbiggen). The quality of the image matters a lot. A really washed out or overexposed snapshot is very hard to work from.
For instance, the following image would be very difficult to sketch. First, the face is far to small to draw anythign accurate. Second, the face is washed out, it's an even smooth tone. NO wrinkles or points of reference are visible. It would be very difficult to translate this face to a drawing or painting.
The following image is better- larger in size and the face is not over exposed. It would work to sketch. There are lots of nice gray tones in the image and it would be easy to pick out dark and light areas to work. (Plus look at that crazy mustache!)
I've mentioned in Challenge! that I love working from the images in Flickr's Commons, and both of these images are from the set "Civil War Faces." There is a lot of history in that set and some amazing facial hair on the men.
When I sketch faces I start first by observing the image close, I make mental notes of where the darkest and lightest area are located. Then I make a light reference drawing and add layers of light gray to build up layers of shadow and end with the darkest darks. This drawing of "Modern Cowboy," shows my method.