functions file

<?php


 if (function_exists('register_nav_menus')) {
        register_nav_menus(
            array(
                'left-nav' => __( 'LeftNav' ),
                'team-level1' => __( 'Team Level 1' ),
                'team-level2' => __( 'Team Level 2' ),
                'team-level3' => __( 'Team Level 3' ),
                'team-level4' => __( 'Team Level 4' ),
            )
        );
    }


// activates Featured Image function
    add_theme_support( 'post-thumbnails' );



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 );
    }
}


add_filter( 'wp_nav_menu_args' , 'my_add_menu_descriptions' );
function my_add_menu_descriptions( $args ) {
    if ( ( $args['theme_location'] == 'team-level1' ) || ( $args['theme_location'] == 'team-level2' ) || ( $args['theme_location'] == 'team-level3' ) || ( $args['theme_location'] == 'team-level4' ) ) {
        $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;
}
?>