Can’t replace default Gravatar

I’m trying to replace the default gravatar from the Gravatar plug-in v1.1 From what I’m reading the code is in the comments.php file. The only code I can find is below. I’m assuming I’d put a url after …40″? I tried that but didn’t work. Any help would be great.

<div class=”gravatar”> </div>
<?php
} else {
?>
<div class=”gravatar”><img src=”http://www.gravatar.com/avatar.php?gravatar_id=&lt;?php echo md5($comment->comment_author_email); ?>&rating=PG&size=40″ /></div>
<?
}
?>