how to add custom thumnail in wordpress

There follows code for wordpress custom thumbnail

<div>
 <a href=”<?php the_permalink() ?>” title=”<?php the_title(); ?>”>
 <?php
     $thumb = get_post_thumbnail_id();
     $img_url = wp_get_attachment_url( $thumb,’full’ );
     $image = aq_resize( $img_url, 600, 250, true );
 ?>
 <img src=”<?php echo $image ?>” width=”600″ height=”250″ alt=”<?php the_title();    ?>”/>
 </a>
 </div>