You will have to take three images.All will be of the same magnitude as the given charge.The first one will be directly below it at same dist from the plate as the given charge.Its sign will also be the same.the second one at similar postion wrt to the other plate but of opp sign.The last one will be of opp sign and will be located at(-l,-l) if (l,l) is the position coordinate of the given charge.