The photographs should be:

• taken within the last six months;
• eye glasses are not allowed in visa photos.
• 50mm (2 inches) square, with the head centered in the frame. The head (measured from the top of the hair to the bottom of the chin) should measure between 25mm to 35mm (1 and 1 3/8 inches) with the eye level between 28mm and 35 mm (1 & 1/8 inch to 1 & 3/8 inches) from the bottom of the photograph;
• in color against a white or off white background. Photographs taken in front of busy, patterned or dark backgrounds are not acceptable;
• unmounted, full face, with the face covering about 50% of the area of the photograph. In general, the head of the applicant, including both face and hair, should be shown from the crown of the head to the tip of the chin on top and bottom, and from hair-line side-to-side. Head-coverings are acceptable only when the applicant’s face is completely exposed.
  The photographs may contain a small (one quarter inch) white border on one side.
  Here is a detailed guide on U.S. visa photography.