core_renderer.php

    public function render_user_picture(user_picture $userpicture) {
        $bill = new bill_murray_picture($userpicture->user);
        return parent::render_user_picture($bill);
    }

}

class bill_murray_picture extends user_picture {

private $numberofbills = 18;

    public function get_url(moodle_page $page, renderer_base $renderer = null) {
            global $OUTPUT;

            $bill = rand(0, $this->numberofbills);
            return $OUTPUT->pix_url('bill'.$bill, 'theme');
    }

}