The post office charges $15 for passport photos. AAA even charges members for passport photos. And in each instance you really get only one try.
Instead, take your picture with your own digital camera. Find one you like and then upload it to ePassportPhoto.com. The site will help you size the picture properly (for the U.S. that means a 2" x 2") and then you can print it off on your own printer.
The site gives full instructions on how to take the picture (lots of room around head and shoulders) and will verify it meets the required standards. It looks easy and it's free.