Posting a post's Thumbnail Image outside of WordPress install folder

I’m using the following code to pull the blog posts outside the blog install

?php
require('/home/USERNAME/public_html/blog/wp-blog-header.php');
?>
<?php query_posts('showposts=5'); ?>
<?php while (have_posts()) : the_post(); ?>
<h3><a>" rel="bookmark" title="Read The Entire Post"><?php the_title(); ?></a></h3>
<p><?php the_excerpt(); ?></p>
<?php endwhile;?>

I want to also pull the thumbnail image out of wordpress too. How can i do that? The thumbnail image is a custom field.

Thanks