The variable "$comment_author_email" would be a string of a valid email address. If the email isn't in the Gravatar database, it will return a default graphic. "$comment_author_email" is the default WordPress variable that populates from a cookie for people who have previously commented (if the theme supports it).
<?php $gravatar_link = 'http://www.gravatar.com/avatar/' . md5($comment_author_email) . '?s=32'; echo '<img src="' . $gravatar_link . '" />'; ?>