Gravatar Plugin, Default Grafik geht nicht

Wer weiss was falsch ist, meine Defaultgrafik wir nicht angezeigt !

<?php if (function_exists('gravatar')) {
if ('' != get_comment_author_url()) {
echo "<a href='$comment->comment_author_url' title='Visit $comment->comment_author'>";
} else {
echo "<a href='http://www.gravatar.com' title='Erstelle dir dein eigenes Gravatar auf http://www.gravatar.com!'>";
}
echo "<img src='";

if ('' == $comment->comment_type) {
echo gravatar($comment->comment_author_email);
} elseif ( ('trackback' == $comment->comment_type) || ('pingback' == $comment->comment_type) ) {
echo gravatar($comment->comment_author_url);
}
echo "' default='http://www.romankeller.ch/blog/wp-content/gravatar.png' alt='Kein eigenes Gravatar' class='gravatar' width='30' height='30' /></a>";
} ?>