1055 'milano_zc1.p.products_image' isn't in GROUP BY
in:
[select p.products_id, p.products_image from zen_orders_products opa, zen_orders_products opb, zen_orders o, zen_products p where opa.products_id = '18' and opa.orders_id = opb.orders_id and opb.products_id != '18' and opb.products_id = p.products_id and opb.orders_id = o.orders_id and p.products_status = 1 group by p.products_id order by o.date_purchased desc limit 6]