Костя

// Вставляете его в файл function.php
// Add the Meta Box
function add_custom_meta_box() {
    add_meta_box(
        'custom_meta_box', // $id
        'Custom Meta Box', // $title
        'show_custom_meta_box', // $callback
        'post', // $page
        'normal', // $context
        'high'); // $priority
}
add_action('add_meta_boxes', 'add_custom_meta_box');

function show_custom_meta_box( $post) {
// Use nonce for verification
echo '<input type="hidden" name="custom_meta_box_nonce" value="'.wp_create_nonce(basename(__FILE__)).'" />';
   
// get value of this field if it exists for this post  
 $meta = get_post_meta($post->ID, 'thum_disable', true);

echo '<input type="checkbox" name="thum_disable" id="thum_disable" ' . checked($meta, "on"); . ' value="on" />
        <label for="thum_disable">Отключить миниатюру</label>';
}

// Save the Data
function save_custom_meta($post_id) {
        
    // verify nonce
    if (!wp_verify_nonce($_POST['custom_meta_box_nonce'], basename(__FILE__)))
        return $post_id;
    // check autosave
    if (defined('DOING_AUTOSAVE') && DOING_AUTOSAVE)
        return $post_id;
    // check permissions
    if ('page' == $_POST['post_type']) {
        if (!current_user_can('edit_page', $post_id))
            return $post_id;
        } elseif (!current_user_can('edit_post', $post_id)) {
            return $post_id;
    }
     
     $old = get_post_meta($post_id, 'thum_disable', true);
     $new = $_POST['thum_disable'];
        if ($new && $new != $old) {
            update_post_meta($post_id, 'thum_disable', $new);
        } elseif ('' == $new && $old) {
            delete_post_meta($post_id, 'thum_disable', $old);
        }
}
add_action('save_post', 'save_custom_meta');

$thumb_disable = get_post_meta($post->ID, 'thum_disable', true);
if(!empty( $thumb_disable )) { the_post_thumbnail( ); }