TWO RECENT, COLOR PHOTOGRAPHS. Submit two color photographs of you alone, sufficiently recent to be a good likeness of you (normally taken within the last six months), and 2x2 inches in size. The image size measured from the bottom of the chin to the top of your head (including hair) should not be less than 1 inch and not more than 1 3/8 inches. The photographs must be color, clear, with a full front view of your face, and printed on thin paper with plain light (white or off-white) background. They must be capable of withstanding a mounting temperature of 225 Fahrenheit (107 Celsius). Photographs must be taken in normal street attire, without a hat, head covering, or dark glasses unless a signed statement is submitted by the applicant verifying the item is worn daily for religious purposes or a signed doctor's statement is submitted verifying the item is used daily for medical purposes. Photographs retouched so that your appearance is changed are unacceptable. Snapshots, most vending machine prints, and magazine or full-length photographs are unacceptable. Digitized photos must meet the previously stated qualifications and will be accepted for use at the discretion of Passport Services. (Visit our websiteHmmm... Does this mean one can wear a burka on a passport photo?? Excuse me???
There is a pdf guide to taking passport photos.
See http://travel.state.gov/passport/guide/faq/faq_881.html for their FAQ
Q. Can hats or religious headgear be worn for the photo?
A. Unless worn daily for religious reasons, all hats or headgear should be removed for the photo. In all cases, no item or attire should cover or otherwise obscure any part of the face.
Bold is added.
The people who wrote the instructions probably have in mind a Jewish yarmulke or a Sikh turban, neither of which obscure the face.