Voyager images of Jupiter show numerous luminous spots on the nightside. These spots have been attributed to lightning superbolts because of their extreme brightness and because of their transitory nature. In order to determine the vertical location of the lightning a comparison is made between the measured spot sizes for single flashes, with intensity distributions calculated from various assumptions about the altitude of the flash. A Monte Carlo model is used to describe the light scattering in the clouds, aerosols, and gases of the Jovian atmosphere. Best agreement between the measurements and the model predictions is obtained when the lightning activity is assumed to occur in a lower cloud centered at 5 bars. A sensitivity study shows that the assumed position of the lightning within the cloud, the optical thickness of the clouds, and variations in the scattering an absorption properties of the clouds have only a minor influence on the predictions of the spot sizes. Because the model predictions of the spot sizes for the lower cloud and ammonia cloud lightning activity must be in the lower cloud rather than in the ammonia cloud. The lower cloud is expected to composed of water or a mixture of water and ammonium hydrosulfide.