Ottenere l'immagine in evidenza per un post type personalizzato

Ho bisogno di ottenere l'URL per l'immagine in evidenza (thumbnail) del mio Custom Post Type, che si chiama "slider". Ho definito il post type in functions.php:

/* Custom post type */
add_action('init', 'slider_register');

function slider_register() {

    $labels = array(
        'name' => __('Slider', 'post type general name'),
        'singular_name' => __('Slider Item', 'post type singular name'),
        'add_new' => __('Add New', 'portfolio item'),
        'add_new_item' => __('Add New Slider Item'),
        'edit_item' => __('Edit Slider Item'),
        'new_item' => __('New Slider Item'),
        'view_item' => __('View Slider Item'),
        'search_items' => __('Search Slider'),
        'not_found' =>  __('Nothing found'),
        'not_found_in_trash' => __('Nothing found in Trash'),
        'parent_item_colon' => ''
    );

    $args = array(
        'labels' => $labels,
        'public' => true,
        'publicly_queryable' => true,
        'show_ui' => true,
        'query_var' => true,
        'menu_icon' => get_stylesheet_directory_uri() . '/image/slider.png',
        'rewrite' => true,
        'capability_type' => 'post',
        'hierarchical' => false,
        'menu_position' => null,
        'supports' => array('title','editor','thumbnail')
        );

    register_post_type( 'slider' , $args );
    flush_rewrite_rules();
}

add_filter("manage_edit-slider_columns", "slider_edit_columns");

function slider_edit_columns($columns){
    $columns = array(
        "cb" => "<input type='checkbox' />;",
        "title" => "Portfolio Title",
    );

    return $columns;
}

Il mio codice è:

<!-- Slider -->
<?php
    $args = array(
        'post_type'=> 'post',
        'post_status' => 'publish',
        'order' => 'DESC',
        'tax_query' => array(
            array(
                'post-type' => array('post', 'slider')
            )
        )
    );
    $query = new WP_Query($args);
    if( $query -> have_posts() ) {
?>
    <div id="slider_area">
        <div class="slider">
            <a href='#' class="prev"><i class="fa fa-angle-double-left"></i></a>
            <a href='#' class="next"><i class="fa fa-angle-double-right"></i></a>
            <ul class="slider_list">
                <?php
                    while($query->have_posts()) : $query->the_post();
                        if(has_post_thumbnail()) { ?>
                            <li>
                                <?php the_post_thumbnail(); ?>
                            </li>
                        <?php }
                         elseif($thumbnail = get_post_meta($post->ID, 'image', true)) { ?>
                             <li>
                                 <img src="<?php echo $thumbnail; ?>" alt="<?php the_title(); ?>" title="<?php the_title(); ?>" />
                             </li>
                     <?php } endwhile;
                 ?>
            </ul>
        </div>
    </div>
<?php } ?>

Qual è il problema? Qualcuno può aiutarmi? Grazie per l'aiuto.