Thin films of La0.7Sr0.3MnO3 were grown by molecular beam epitaxy on (001)LaAlO3 crystals. High resolution X-ray diffraction analysis proves the presence of twins in the films at room temperature, showing that the twin structure of the substrate which forms at the Pm¯3m → R¯3c ferroelastic transition at TF = 813 K served as a template for the film microstructure. Magnetic measurements indicate a thermomagnetic irreversibility which is ascribed to the quenched disorder related to twinning and discussed in terms of coexisting ferromagnetic and spin disordered regions connected with the undeformed domain cores and strained domain walls respectively.