It’s not images, its fontawesome icons, and the fontawesome library is not being called. so you can add the below code to your theme functions.php
add_action( 'wp_enqueue_scripts', 'enqueue_load_fa' );
function enqueue_load_fa() {
wp_enqueue_style( 'load-fa', 'https://maxcdn.bootstrapcdn.com/font-awesome/4.7.0/css/font-awesome.min.css' );
}
thats how the current icons for the menus will be showing.