Menu_With_Description.php

<?php

class Menu_With_Description extends Walker_Nav_Menu {
	function start_el( &$output, $item, $depth, $args ) {
		global $wp_query;

		$indent = ( $depth ) ? str_repeat( "t", $depth ) : '';

		$class_names = $value = '';

		$classes = empty( $item->classes ) ? array() : (array) $item->classes;

		$class_names = join( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item ) );
		$class_names = ' class="' . esc_attr( $class_names ) . '"';

		$output .= $indent . '<li id="menu-item-' . $item->ID . '"' . $value . $class_names . '>';

		$attributes = ! empty( $item->attr_title ) ? ' title="' . esc_attr( $item->attr_title ) . '"' : '';
		$attributes .= ! empty( $item->target ) ? ' target="' . esc_attr( $item->target ) . '"' : '';
		$attributes .= ! empty( $item->xfn ) ? ' rel="' . esc_attr( $item->xfn ) . '"' : '';
		$attributes .= ! empty( $item->url ) ? ' href="' . esc_attr( $item->url ) . '"' : '';

// get user defined attributes for thumbnail images
		$attr_defaults = array(
			'class' => 'nav_thumb',
			'alt'   => esc_attr( $item->attr_title ),
			'title' => esc_attr( $item->attr_title )
		);
		$attr          = isset( $args->thumbnail_attr ) ? $args->thumbnail_attr : '';
		$attr          = wp_parse_args( $attr, $attr_defaults );

		$item_output = $args->before;

// thumbnail image output
		$item_output .= ( isset( $args->thumbnail_link ) && $args->thumbnail_link ) ? '<a' . $attributes . '>' : '';
		$item_output .= apply_filters( 'menu_item_thumbnail', ( isset( $args->thumbnail ) && $args->thumbnail ) ? get_the_post_thumbnail( $item->object_id, ( isset( $args->thumbnail_size ) ) ? $args->thumbnail_size : 'thumbnail', $attr ) : '', $item, $args, $depth );
		$item_output .= ( isset( $args->thumbnail_link ) && $args->thumbnail_link ) ? '</a>' : '';

// menu link output
		$item_output .= '<a' . $attributes . '>';
		$item_output .= $args->link_before . apply_filters( 'the_title', $item->title, $item->ID ) . $args->link_after;

// menu description output based on depth
		$item_output .= ( $args->desc_depth >= $depth ) ? '<br /><span class="sub">' . $item->description . '</span>' : '';

// close menu link anchor
		$item_output .= '</a>';
		$item_output .= $args->after;

		$output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );
	}
}