Add description by menu location.

my_add_menu_descriptions.php

<?php
add_filter( 'wp_nav_menu_args', 'my_add_menu_descriptions' );
function my_add_menu_descriptions( $args ) {
	if ( $args['theme_location'] == 'primary' ) {
		$args['walker']         = new Menu_With_Description;
		$args['desc_depth']     = 0;
		$args['thumbnail']      = true;
		$args['thumbnail_link'] = false;
		$args['thumbnail_size'] = 'nav_thumb';
		$args['thumbnail_attr'] = array( 'class' => 'nav_thumb my_thumb', 'alt' => 'test', 'title' => 'test' );
	}

	return $args;
}