In a zero-average refractive index metamaterial the light propagation is forbidden and a narrow full photonic bandgap is open in correspondence of the frequency where it vanishes and the refractive index is averaged over the volume. However, it should be underlined that Fabry-Pérot resonances can open a propagation state inside such a particular type of bandgap. Based on a photonic crystal exhibiting a negative refractive index, it was found that such Fabry-Pérot resonant states allow images transmission with subwavelength details.