1. A computer-implemented method for providing challenge graphics, the method comprising acts of: receiving user behavior information indicative of a behavior of a user, wherein the user behavior information comprises user behavior information other than whether the user provided a correct response to a prior challenge graphic; adjusting a level of trust worthiness assigned to the user, wherein the level of trustworthiness is adjusted at least in part by analyzing the user behavior information other than whether the user provided a correct response to a prior challenge graphic; with at least one server, generating and serving to a user a present challenge graphic, wherein: the present challenge graphic is selected based on the level of trust worthiness assigned to the user; and the present challenge graphic is generated randomly and/or pseudo-randomly.