Building Gravatar Code

If you look at the Gravatar API you will see that you can call a user specific and specify a default so here a built proof way to get the the image url in your code.

$email = $comment->comment_author_email;
$size = 60;
$rating = get_option('avatar_rating');
$default = urlencode( 'http://www.gravatar.com/avatar/ad516503a11cd5ca435acc9bb6523536?s=" . $size );
$grav_url = "http://www.gravatar.com/avatar/'. md5( strtolower( trim( $email ) ) ). '?s=" . $size . "&r=" . $rating . "&d=' . $default;
?>
<div class="gravatars" style="background: url(<?php echo $grav_url ?>)"></div>

and you have no need for conditional checks.