First idea, use a background image via css for the first level.
If you have more then one picture (for every category one) please use IDs for the navigation.
{NAV_LIST_UL:F,ID,,act_path,active, navID }
{NAV_LIST_UL:menu_type, start_id, max_level, class_path, class_active, ul_id_name , wrap_ul ...