Head Position The head height or facial region size (measured from the top of the head, including the hair, to the bottom of the chin) must be between 50 percent and 69 percent of the image's total height. The eye height (measured from the bottom of the image to the level of the eyes) should be between 56 percent and 69 percent of the image's height.
Image File Format:The image must be in the Joint Photographic Experts Group (JPEG) format.
Image File Size:The maximum image file size is 240 kilobytes (240 KB).
Image Resolution and Dimensions:Minimum acceptable dimensions are 600 pixels (width) x 600 pixels (height). Image pixel dimensions must be in a square aspect ratio (meaning the height must be equal to the width).
Image Color Depth: Image must be in color (24 bits per pixel). 24-bit black and white or 8-bit images will not be accepted.

Source: http://travel.state.gov/pdf/1318-DV2012Instructions-ENGL.pdf