<?php $args = array(
    'post_type'      => 'attachment',
    'post_parent'    => $post->ID,
    'post_status'    => 'inherit',
    'post_mime_type' => 'image',
);?>



<ul>
<?php $posts = query_posts( $query_string . 'order=asc' ); ?>

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

<?php $attachments = new WP_Query( $args );
$attachments->get_posts();

$output = '';
foreach( $attachments->posts as $attachment ) {
    $image_src = wp_get_attachment_image_src( $attachment->ID, 'large');
    $output = '<a href="'. $image_src[0] .'" rel="lightbox[post-1]" class="hidden"></a>';
}
?>

<?php
$image = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'single-post-thumbnail' );
?>
<a href="<?php $image_src[0]; ?>" rel="lightbox[post-1]">
<?php the_post_thumbnail( 'thumbnail-size'); ?>
</a>




<?php endwhile; endif; ?>