Pass $item->description to start_lvl

// Get menu description as global variable
function add_menu_description( $item_output, $item, $depth, $args ) {
    global $description;
    $description = $item->post_content;
    return $item_output;
}
add_filter( 'walker_nav_menu_start_el', 'add_menu_description', 10, 4);

// Submenu walker to add image
class submenu_walker extends Walker_Nav_Menu {
    function start_lvl( &$output, $depth = 0, $args = array() ) {
        if(0 == $depth) {
            global $description;
            $output .= '<ul class="sub-menu">'.
                '<li class="row marginbottom1">'.
                    '<div class="col-sm-8 col-lg-9">'.
                    '<div class="menu-title">'.
                            //$item->title.
                        '</div>'.
                        '<div class="menu-description">'.
                            $description.
                        '</div>'.
                    '</div>'.
                    '<div class="col-sm-4 col-lg-3"><div class="menu-image img-rounded"></div></div>'.
                '</li>';
        } else {
            $output .= '<ul class="sub-menu">';
        }
    }
}