The voltage dependence of photoinjected currents in SiO2 films for electric fields less than about 106 V/cm exhibits anomalous behavior. It is shown that the physical mechanism responsible for this behavior is the scattering of photoinjected electrons in the SiO2 image force potential well between the emitter and the potential maximum. A previously published model attributing the effect to electron trapping in SiO2 is shown to be inconsistent with the experimental results. A theoretical model is presented and the voltage dependence of photocurrents is derived including the effects of scattering and barrier lowering. Experimental results from MIS structures using thermally grown SiO2 are found to be in excellent agreement with theoretical predictions when a 34 Å mean free path for scattering in SiO2 is used in the model.