1. A computer-implemented method for authenticating users of an online system based on cameras associated with the user, the method comprising: receiving a request from the user for access to the online system; responsive to the request from the user, sending a request to the user to authenticate the user's account by providing one or more additional images taken from the camera; receiving the requested one or more additional images from the user; retrieving a signature of a camera associated with the user, the signature based on images captured by the camera previously received; comparing the one or more additional images to the signature of the camera; and responsive to the one or more additional images matching the signature of the camera, providing the user with the requested access to the online system.