List of posts with thumbnails

Hi I’m trying to output a list of posts within a certain category with an image above the title, here’s where I am:

<?php
 global $post;
 $myposts = get_posts('category=this_months_news');
 foreach($myposts as $post) :
		the_attachment_link( 13, false, array(48, 48));

 ?>
   <li><a href="<?php the_permalink(); ?>" title="<?php the_title(); ?>"><?php the_title(); ?></a></li>
 <?php endforeach; ?>

I’ve added this to the sidebar as a menu. It’s showing the post titles as links but says ‘missing attachment’ where the image should be?

Can anyone tell me what I’m doing wrong please?