if (!function_exists ( 'display_series_thumbs' ) )
{
    function display_series_thumbs()
    {
        // check if the thumbnails exist
        if ( has_post_thumbnail() ) {
            the_post_thumbnail( $post->ID, 'post-series-thumbnail',array('title' => "") );
        } else {
        // if they don't, use the default one
        ?>
            <img class="post-series-default-thumb" src="#" alt="<?php the_title(); ?>" />
        <?php
        }
    }
}