我需要获取自定义帖子类型缩略图的URL，我的自定义帖子类型名称是slider。我已经定义了函数。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;
}

<!-- 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 } ?>