Also most photo shops and studios know what is required and the format (I worked in one once that had a jig that was specifically sized to cut out US passport sized photos) and will usually guarantee that the photos will pass the state department's requirements for positioning, etc. (and if not they'll at minimum retake them for you) whereas if you get it done at the post office, they don't seem to really care, and if your photos get rejected, you'll get an apathetic shrug in response.