Gravatare werden nicht angezeigt

function top_commentator_all() {    global $wpdb;     $i = 0;    ?>    <div class="walloffame_container">                <?php $topcommentator_query="SELECT comment_author, comment_author_email, comment_type, SUBSTRING_INDEX(REPLACE(comment_author_url, 'www.', ''), '/', 3) as top_commentator, COUNT(*) as top_num FROM $wpdb->comments        WHERE comment_approved='1'         AND comment_author_url IS NOT NULL        AND comment_author_url != ''        AND comment_author_email NOT LIKE '@themenfreund.de%'        AND comment_author_url NOT LIKE 'http://facebook.com/%'        AND comment_author_url NOT LIKE 'http://themenfreund.de%'        AND comment_author_url NOT LIKE 'http://twitter.com%'        AND comment_type NOT LIKE 'pingback' OR 'trackback'        GROUP BY top_commentator        Order by top_num DESC, comment_date ASC        LIMIT 18";    $topcommentator_result = $wpdb->get_results($topcommentator_query);     if ( !empty($topcommentator_result) ) {        foreach ($topcommentator_result as $comment) :            $co_aut = $comment->top_commentator;            $co_aut = str_ireplace("http://", "", $co_aut); ?>            <div class="walloffame_stage" style="cursor: pointer;">            <div class="walloffame_avatar">             <?php            echo '<a href="'.$comment->top_commentator.'/" target="_blank">'.get_avatar( $comment, 164 ) .'</a>';             ?>            </div>            <div class="walloffame_info">            <span class="walloffame_name" >            <?php            echo $comment->comment_author.' <br />';            ?>            </span>            <span class="walloffame_total">            <?php             echo $comment->top_num.' Kommentare'; ?>            </span>            </div>            </div>            <?php            $i++;            if($i%3==0) { ?>                        <?php            }        endforeach;        $i=0;        }        ?>        </div>            <?php        } ?>