The photo requirements are detailed online. It must be color, 2 x 2 inches, sized so that the head is 1 to 1 3/8 inches from chin to the top of the head, taken in the last 6 months, taken in front of a white background, taken with a full-face view, and with a neutral or natural smile with both eyes open. The website above also provides additional information if you want to take your own, such as the required resolution and a tool to make your own.