Using Gravatar code without the plugin

WPDesigner recently made a post on adding gravatars to comments without the plugin. The code looks like this:

<?php if(!empty($comment -> comment_author_email)) {
$md5 = md5($comment -> comment_author_email);
$default = urlencode('PATH to default image');
echo "<img src='https://www.gravatar.com/avatar.php?gravatar_id=$md5&size=20&default=$default' alt='' />";
}
?>

My question is: how to I construct the path to the default image (images/nopic.gif)? Here is the code I tried, which failed:

$default = urlencode(' . get_bloginfo('template_url') . /images/nopic.gif');

Thanks!