<?php
 if ( has_post_thumbnail()) {
   $large_image_url = wp_get_attachment_image_src( get_post_thumbnail_id(), 'large');
   echo '<a href="' . $large_image_url[0] . '" title="' . the_title_attribute('echo=0') . '" >';
   the_post_thumbnail('thumbnail');
   echo '</a>';
 }
 ?>