Don't listen to any of the above posters.
Wear a suit and tie, go to Sears and get your picture taken in their studio. Pay the extra 15$ or so for minor touch ups, and choose the background you like the most out of all the photos they take of you. The total cost will probably be somewhere between 50-100$.
Looks matter, don't look like a scrub and don't use a tacky passport photo. Look professional with a nice portrait taken by a photographer. Patients should want to look at your photo and say, "Wow, I want this guy working on my teeth." My honest opinion.