Techniques are disclosed for implementing a CAPTCHA access control system based on graphical representations of a watch or other timekeeping device. More particularly, the disclosed CAPTCHA system's request/challenge mechanism employs a graphical representation of a watch whose perturbation from a baseline visual presentation is controlled by a large number of attributes, each of which may assume a number of different values. The use of a large number of display attributes (e.g., 20 or more) and a relatively small number of difficulty levels allows each difficulty level to have an enormous number of possible graphical representation. Such a large number of potential challenge images essentially precludes the likelihood that any automated search for a matching image—providing the ability to correctly respond with certainty to the challenge query—will be successful.