comments' thumbs

<?php $args = array(
        'status' => 'approve',
        'number' => '12');
        $comments = get_comments($args);
        foreach($comments as $comment) {

        $post_id = $comment->comment_post_ID;
        $post_thumb_id = get_post_thumbnail_id($post_id);
        $post_attachment_src = wp_get_attachment_image_src( $post_thumb_id, 'medium');
        $post_thumb_url = $post_attachment_src[0];

?>

        <div class="hb-comment" style="background: url('<?php echo $post_thumb_url; ?>');">
        <h5><?php echo($comment->comment_author);?></h5>
        <p><?php echo substr($comment->comment_content,0,50);?></p>
        </div>

        <?php } ?>