Adding a custom attribute to Wordpress menu items (Themes -> Appearance -> Menus -> Item edition). Extracted from https://github.com/AlphaGit/alphasmanifesto/blob/master/custom_menu_setup.php

customImageUrl.php

<?php
    // retrieve menu item custom url attribute when a 
    add_filter( 'wp_setup_nav_menu_item', 'alphasmanifesto_setup_nav_menu_item' );
    function alphasmanifesto_setup_nav_menu_item($menu_item) {
        $attrName = 'image_url'; // property name
        $genericProperty = "alphasmanifesto_menu_item_$attrName"; // unique property name -- avoid colliding with other plugins/themes
        
        // get it from the database with its id, set it to the menu item
        $menu_item->image_url = get_post_meta($menu_item->ID, $genericProperty, true);
        
        // return the modified menu item
        return $menu_item;
    }
    
    // save menu item custom url attribute
    add_action( 'wp_update_nav_menu_item', 'alphasmanifesto_update_nav_menu_item', 10, 3 );
    function alphasmanifesto_update_nav_menu_item($menu_id, $menu_item_id, $args) {
        $attrName = 'image_url'; // property name
        $genericProperty = "alphasmanifesto_menu_item_$attrName"; // unique property name -- avoid colliding with other plugins/themes

        // if the value is being submitted through post (being saved)
        if ( isset( $_POST[ "menu-item-image-url" ] ) ) {
            // retrieve the one bound to this item ID (see the form generation)
            $value = $_POST[ "menu-item-image-url" ][$menu_item_id];
            
            // save the value to the database with the item id
            update_post_meta( $menu_item_id, $genericProperty, $value );
        } else {
            // if its not being provided, delete it from the database, for that item id            
            delete_post_meta( $menu_item_id, $genericProperty );
        }
    }
?>