Recent Comments how to change the default gravatar

Hi I am using the following script to display my most recent comments it also displays the gravatar as well.

global $wpdb;

$sql = "SELECT DISTINCT ID, post_title, post_password, comment_ID,
comment_post_ID, comment_author, comment_author_email, comment_date_gmt, comment_approved,
comment_type,comment_author_url,
SUBSTRING(comment_content,1,50) AS com_excerpt
FROM $wpdb->comments
LEFT OUTER JOIN $wpdb->posts ON ($wpdb->comments.comment_post_ID =
$wpdb->posts.ID)
WHERE comment_approved = '1' AND comment_type = '' AND
post_password = ''
ORDER BY comment_date_gmt DESC LIMIT 5";

$comments = $wpdb->get_results($sql);
$output = $pre_HTML;

foreach ($comments as $comment) {

	// Determine which gravatar to use for the user
	$email = $comment->comment_author_email;
	$grav_url = "https://www.gravatar.com/avatar.php?gravatar_id=".md5($email). "&size=35";

?>

But it displays the default gravatar avatar I want to use my own default avatar how do I change it?

Hope you can help

thanks