Try this:
<?php
global $wpdb;
$query = "SELECT ID, user_nicename from $wpdb->users ORDER BY user_nicename";
$author_ids = $wpdb->get_results($query);
foreach($author_ids as $author) :
$curauth = get_userdata($author->ID);
if($curauth->user_login !== 'admin') :
$user_link = get_author_posts_url($curauth->ID);
$avatar = 'default';
?>
<div class="autor" title="<?php echo $curauth->display_name; ?>">
<?php if(userphoto_exists($curauth)) userphoto_thumbnail($curauth); else echo get_avatar($curauth->ID, 40); ?>
</div>
<?php endif; ?>
<?php endforeach; ?>
