I have maintained over the years, that for the most part, if you are going to work from a photograph, it ought to be one you take yourself. Suffice to say, I did not take the pic of Jason Lewis that I worked from (but I also know, I would not have had an opportunity to draw him otherwise). The primary reasons you should take your own photo to work from are many. For one thing, you learn more about composition when you are the one behind the camera choosing the light, subject, what will or will not be in the frame, angle, and the list goes on.
Below are several photos where no adjustments have been made. It is better when you are just beginning in photography to concentrate on a complete composition within the frame of the shot, without relying on digital adjustments later in the processing area. It will make a better photographer of you to get in this mindset!
Of course, once you've been at it for a while, you may find you can rescue an otherwise mediocre image by cropping, and other enhancements...
In closing, whether or not you are an artist working from photos, whether or not you want to be a photographer, you really can take better photos by concentrating on what makes a good composition!
I will post again soon with more faces. Until then, take care!