Add Featured Image and Title to wp_nav_menu items

The following code will get you the featured image for 3rd level of your navbar.
You, have to pass the nav-menu id within wp_get_nav_menu_items(). It will get you the menu list (For more details).

<ul>
    <?php
        $navMenu = wp_get_nav_menu_items(9); /*/Pass Nav Menu_id or Name*/
        $previousMenuParent = $level = 0;
        foreach ($navMenu as $menu) {

            if($menu->menu_item_parent == 0)
            {
                $level = 0;
                echo '<li><a href="'. $menu->url .'">'. $menu->title .'</a>';
            }
            elseif($menu->menu_item_parent != '' && $menu->menu_item_parent != $previousMenuParent)
            {
                $level++;
                echo '<ul class="submenu">';
                echo '<li><a href="'. $menu->url .'">'. $menu->title .'</a>';
                $previousMenuParent = $menu->menu_item_parent;
            }
            elseif($previousMenuParent == $menu->menu_item_parent)
            {
                echo '</li><li><a href="'. $menu->url .'">';
                if($level == 3)
                {
                    echo get_the_post_thumbnail($menu->ID);
                }
                echo $menu->title .'</a>';
            }
        }
    ?>
    </li>
</ul>