gistfile1.php

class My_Walker_Nav_Menu extends Walker_Nav_Menu {
       //ここからカスタマイズのコードを書く
	function start_el( &$output, $item, $depth = 0, $args = array(), $id = 0 ) {
		global $wp_query;
		$indent = ( $depth ) ? str_repeat( "\t", $depth ) : '';

		$class_names = $value = '';

		$classes = empty( $item->classes ) ? array() : (array) $item->classes;
		$classes[] = 'menu-item-' . $item->ID;

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

		$id = apply_filters( 'nav_menu_item_id', 'menu-item-'. $item->ID, $item, $args );
		$id = $id ? ' id="' . esc_attr( $id ) . '"' : '';

		$output .= $indent . '<li' . $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        ) .'"' : '';

		$item_output = $args->before;
		$item_output .= '<a'. $attributes .'>';
		$item_output .= get_the_post_thumbnail($item->object_id, array(100,100));
		$item_output .= '</a>';
                if (empty($args->remove_title)) {
                    $item_output .= '<a'. $attributes .'>';
                    $item_output .= $args->link_before . apply_filters( 'the_title', $item->title, $item->ID ) . $args->link_after;
                    $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_nav_menu_args');
function my_nav_menu_args($args) {
    $args = (object)$args;
    $args->walker = new My_Walker_Nav_Menu;
    return $args;
}