Pulling upload thumbnails

I have the following code to pull the auto generate thumbnails to display in an archive page – it works fine in my local server but not now i have uploaded to the web. Any ideas?

<ul>

 <?php query_posts('cat='.get_query_var('cat').'&order=ASC'); ?>

	<?php if (have_posts()) : ?>

		<?php while (have_posts()) : the_post(); ?>

        <?php
//Get images attached to the post

$args = array(
	'post_type' => 'attachment',
	'post_mime_type' => 'image',
	'numberposts' => -1,
        'order' => 'DESC',
	'post_status' => null,
	'post_parent' => $post->ID
);
$attachments = get_posts($args);
if ($attachments) {
	foreach ($attachments as $attachment) {
		$img = wp_get_attachment_thumb_url( $attachment->ID );
                break;
        }
}
?>

            <li>
            	<img src="<?php echo $img; ?>" alt="" />
                <h2 class="small"><a href="<?php the_permalink() ?>"><?php the_title(); ?></a></h2>
            </li>                

		<?php endwhile; ?>

 	  <?php endif;?> 

 </ul>