My personal advice: Don't use an existing pic, unless you've had reason to get business shots done lately, for a company web site or the like. Males: Put on a nice collared shirt, strongly consider a tie, shave, do this within a few days of having had a haircut. Women: Nice blouse, nice hair (but nothing crazy like an up-do), simple makeup. Basically dress like you're going to a business casual interview. You don't have to go to a studio to get the pic done, a friend or family member and a digital camera in front of a neutral colored wall will do. Take a couple dozen shots and pick the best one. This whole process should take 15-30 minutes. With all the other time and effort you've invested in preparing your med school app, it seems crazy to me to try to save a few minutes at this point in the game. Remember, a good photo is not likely to help you at all, but I'd be willing to bet any amount of money that a bad one will definitely hurt you.
If you're looking at this thread in a few years, ignore the above and only read this bit:
Unless you're applying to matriculate in 2015, when I'm planning on it. In that case, find that pic of you doing a keg stand in a Speedo/bikini and send it in-the adcoms will think you're fun and social!