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>