How to show thumbnails with php

I’m having an issue with a featured properties section of a web site that was built for a client. The web site is http://chuckwardrealestate.com/. The issue I’m having is that when the client adds a new photo in the post as a featured image instead of showing up on the web site as a thumbnail it’s showing up in it’s full size in the featured properties unlike the other images. The thumbnail size is selected when they add the picture to the featured image. The only difference I can see is that the images that are already there as thumbnails have been resized to thumbnails before being added to the web site. I don’t want the client to have to resize their images. Is there a piece of code that’s missing that would resize their images to thumbnails for them for the featured properties section?This is the code that’s in the editor right now. I’m almost thinking about scraping this and using the Nextgen gallery plugin. I just want to check here first.

[mod: please read and folow the forum guidelines for posting code]

<div class=”bordercontent”><h2> Featured Properties </h2></div>

<div class=”topleftimagecontent”>

<?php query_posts(‘cat=4&posts_per_page=6’);
if (have_posts()) : while (have_posts()) : the_post(); ?>
<div class=”topFeaturedimage”>
<div class=”Featuredimage”> <?php the_post_thumbnail(); ?></div>
<div class=”Featuredtitle”> <p><?php the_title(); ?></p></div>

</div>
<?php endwhile; endif; ?>
<?php wp_reset_query(); ?>
</div>
</div>
</div>