The recently developed mercury Resonance Ionization Imaging Detector (RIID) has potential in many fields of imaging science. However, when high accelerating voltages (>5kV) are employed in the initial stage of detection, severe spatial image distortions are observed. This effect is most noticeable after extended periods of RIID operation (10-25 min). In this study, we attribute these distortions to surface charging effects of the input window. The magnitude of the surface charging, and thus the degree of image distortion, is shown to be related to the accelerating voltages applied. Methods for neutralizing the charging effects are discussed.