Adding Thumbnails to The Loop outside of WordPress

I want to display my recent WordPress posts outside of wordpress on my custom website. I am able to do this pretty well, but my question is how can I add a thumbnail to the posts? This is my current code:

<?php while (have_posts()): the_post(); ?>
<h2><?php the_title(); ?></h2>
<?php the_excerpt(); ?>
<p><a href="<?php the_permalink(); ?>">Read more...</a></p>
<?php endwhile; ?>