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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).

  $gravatar_link = '' . md5($comment_author_email) . '?s=32';
  echo '<img src="' . $gravatar_link . '" />';