From http://blog.escapecreative.com/wordpress-display-all-images-attached-to-post-page/

Display All Attached Images In Post Loop

<?php
	$images =& get_children( array (
		'post_parent' => $post->ID,
		'post_type' => 'attachment',
		'post_mime_type' => 'image'
	));

	if ( empty($images) ) {
		// no attachments here
	} else {
		foreach ( $images as $attachment_id => $attachment ) {
			echo wp_get_attachment_image( $attachment_id, 'thumbnail' );
		}
	}
?>