Default Gravatar not showing for pings

Question

Here is my comments block:

<div id="comments">
    <?php if (have_comments()) : ?>
        <h3><?php printf(_n('1 comment', '%1$s comments', get_comments_number()), number_format_i18n( get_comments_number() ), '' ); ?></h3>
        <div class="comment_list">

            <?php $comments_by_type = &separate_comments($comments); ?>
            <?php if ( !empty($comments_by_type['comment']) ) : ?>
            <ol>
                <?php wp_list_comments(array('callback' => 'commentslist', 'type' => 'comment')); ?>
            </ol>
            <?php endif; ?>

            <?php if ( ! empty($comments_by_type['pings']) ) : ?>
            <h3 id="pings">Pingbacks/Trackbacks</h3>
            <ol class="pinglist">
            <?php wp_list_comments(array('callback' => 'commentslist', 'type' => 'pings')); ?>
            </ol>
            <?php endif; ?>
        </div>
    <?php endif; // end have_comments() ?>
    </div>

Here is my wp_list_comments callback:

function commentslist($comment, $args, $depth) {
    $GLOBALS['comment'] = $comment;
        require_once(ABSPATH . WPINC . '/registration.php');
        if ($comment->user_id || email_exists($comment->comment_author_email)){
            //comment by registered user
            $avatar = '/images/bird_comments_big.png';
            }else{
            //comment by none registered user
            $avatar = '/images/bird_comments_pink.png';
            }
        ?>
    <li>
        <div id="comment-<?php comment_ID(); ?>" <?php comment_class(); ?>>
            <table>
                <tr>
                    <td>
                        <?php echo get_avatar($comment, 70, get_bloginfo('template_url').$avatar); ?>
                    </td>
                    <td>
                        <div class="comment-meta">
                            <?php printf(__('<p class="comment-author"><span>%s</span> says:</p>'), get_comment_author_link()) ?>
                            <?php printf(__('<p class="comment-date">%s</p>'), get_comment_date('M j, Y')) ?>
                            <?php comment_reply_link(array_merge($args, array('depth' => $depth, 'max_depth' => $args['max_depth']))) ?>
                        </div>
                    </td>
                    <td>
                        <div class="comment-text">
                            <?php if ($comment->comment_approved == '0') : ?>
                                <p><?php _e('Your comment is awaiting moderation.') ?></p>
                                <br/>
                            <?php endif; ?>
                            <?php comment_text() ?>
                        </div>
                    </td>
                </tr>
            </table>
         </div>
<?php
}

It seems to that wp_gravatar should call up the non-registered user image due to a lack of an e-mail address for pings, but instead its returning nothing.

Answer 1

By default WordPress does not display an avatar for a pingback or a trackback - do they even contain an e-mail address? You can add these to the get_avatar_comment_types filter if you want to change this.