post thumbnail trouble

Using the following piece of code for a client, trying to echo out the featured image so it shows up. All we get is the permalink title and the excerpt, no thumbnail. code is as follows:

$permalink = get_permalink(); // Get the post's permalink
    $thumbnail = the_post_thumbnail(array(60,60), array("class" => "post_thumbnail"));

    echo '<li><div class="service">';

    echo '<h3><a href="' . $permalink . '">';
        echo $post->post_title . ': ';
    echo '</a></h3>';

    echo $thumbnail;

    echo the_excerpt();

    echo '</div></li>';