Include featured images of posts and pages in WordPress menus

functions.php

add_filter('wp_nav_menu_objects', 'ad_filter_menu', 10, 2);

function ad_filter_menu($sorted_menu_objects, $args) {
    if ($args->menu->slug != 'home-page-slider')
        return $sorted_menu_objects;

    // edit the menu objects
    foreach ($sorted_menu_objects as $menu_object) {
        // searching for menu items linking to posts or pages
        // can add as many post types to the array
        if ( in_array($menu_object->object, array('post', 'page', 'any_post_type')) ) {
            // set the title to the post_thumbnail if available
            // thumbnail size is the second parameter of get_the_post_thumbnail()
            $menu_object->title = has_post_thumbnail($menu_object->object_id) ? get_the_post_thumbnail($menu_object->object_id, 'thumbnail') : $menu_object->title;
        }
    }

    return $sorted_menu_objects;
}