<?php
if ( has_post_thumbnail() ) {
      $large_image_url = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'full' );
      echo '<a href="' . $large_image_url[0] . '" title="' . the_title_attribute( 'echo=0' ) . '" >';
      echo get_the_post_thumbnail( $post->ID, 'medium' );
      echo '</a>';
}
?>