[@_ck_ Participant 15 years, 10 months ago]

Showing the gravatar for topic starter and last poster would be something like this (untested)

Topic started by

<?php echo "<img src='http://www.gravatar.com/avatar.php?gravatar_id=".md5(bb_get_user_email($topic->topic_poster))."'> $topic->topic_poster_name"; ?>

Last Poster

<?php echo "<img src='http://www.gravatar.com/avatar.php?gravatar_id=".md5(bb_get_user_email($topic->topic_last_poster))."'> $topic->topic_last_poster_name"; ?>