Use the theme's avatar as the default if Gravatar doesn't have one.

modules/comment/views/admin_block_recent_comments.html.php

@@ -2,10 +2,13 @@
 <ul>
   <? foreach ($comments as $comment): ?>
   <li>
+    <img width="40" height="40"
+         src="<?= $comment->author()->avatar_url(40, $theme->url("images/avatar.jpg", true)) ?>"
+         class="gAvatar" alt="<?= $comment->author_name() ?>" />
     <?= date("Y-M-d H:i:s", $comment->created) ?>
-    <?= t("{{author_name}} said {{comment_text}}",
-          array("author_name" => "<a href=\"#\">$comment->author</a>",
-                "comment_text" => "<i>\"" . text::limit_words($comment->text, 50) . "\"</i>")); ?>
+    <?= t("<a href=#>{{author_name}}</a> said <i>{{comment_text}}</i>",
+          array("author_name" => $comment->author_name(),
+                "comment_text" => text::limit_words($comment->text, 50))); ?>
   </li>
   <? endforeach ?>
 </ul>