The propagation of the high-spin (HS) → low-spin (LS) relaxation at 53 K in a single crystal of the iron (II) spin-crossover compound [Fe(ptz)6](BF4)2 was followed by photography, after inducing the local photoexcitation to the metastable HS state at 20 K using the single wavelength (457 nm Ar± ion laser) irradiation. The photoinduced formation of the HS—LS patterns with a characteristic diameter of some 0.1 mm was observed to occur inhomogeneously at a macroscopic scale already during photoexcitation. The contrast between the HS (transparent) and the LS (purple) regions was amplified during relaxation. The effect is described in terms of a transient instability, for which a microscopic model in the mean-field approximation is proposed. The mechanism for the development of patterns at the macroscopic scale is discussed.