[Plugin: Yet Another Related Posts Plugin] Problem with post thumbnails

Hi!
For me its not possible, to add hard-cropped Post-Thumbnails of the related Posts on my site.

For example: http://wp.ludwig-jahn-design.de/kultur/filme/die-blechtrommel/#aehnlich

The thumbnails are shown, but not hard-cropped… My Code in the yarpp-Template:

<?php if ($related_query->have_posts()):?>
<div id="aehnlich">
	<h5>Beiträge, die Sie auch interessieren könnten</h5>
	<?php while ($related_query->have_posts()) : $related_query->the_post(); ?>
		<?php if (has_post_thumbnail()):?>
		<div class="aehnlich_pic">
			<a href="<?php the_permalink() ?>" title="<?php the_title_attribute(); ?>">
			<?php the_post_thumbnail('related'); ?>
			</a>
		</div>
		<?php endif;?>
	<?php endwhile; ?>
</div>
<?php endif; ?>

In the functions.php there is the line for resizing and cropping the thumbnails:

add_image_size( 'related', 72, 72, true);

Can you help me out?

Greetings from Germany
lundj