Unlimits

how do i select only people has photos on this code below :

<?php
function contributorsbottom() {
global $wpdb;

$authors = $wpdb->get_results("SELECT ID, user_nicename  from $wpdb->users ORDER BY RAND() LIMIT 12");

foreach($authors as $author) {

?>

<a title="<?php the_author_meta('display_name', $author->ID);?>" href="<?php get_bloginfo('url')?>/members/<?php the_author_meta('user_nicename', $author->ID); ?>"><?php echo get_avatar($author->ID,50); ?></a>

<?

}
}

?>

<?php contributorsbottom(); ?>

This code is very good
if u know, tell me as soon as u can.
thank you.

User profile images, show all users profile images, has some wordpress command to do this ?