We present the observation of the manipulation of modulational instability in a nonlinear dissipative system by a periodic photonic lattice. We use a setup based on a photorefractive BaTiO3 crystal in a single feedback mirror configuration leading to the formation of hexagonal patterns. Additionally, we impose an optical lattice to induce one or two-dimensional photonic band-gap structures with variable parameters. We show that by varying the lattice periodicity, thus adjusting the transverse spatial frequencies associated to the bandgap, we can induce patterns of particular symmetry or suppress the modulational instability when the position of the lattice bandgap coincides with the instability gain.