/***** Images in main TOC ****/

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

function add_menu_images( $items, $args ) {

    if ($args->menu == "Main_TOC") {
      $img_array = array();
      foreach ( $items as $item ) {
        if (!$item->menu_item_parent) {
            $img_url = get_the_post_thumbnail_url($item->object_id);
        }
      }
      $args->items_wrap = '<ul id=%1$s class=%2$s style="background-image: url(' . $img_url . ')">%3$s</ul>';


    } // end if main TOC

    return $items;
}