Processes an image by adding it to accumulated state.

def _ProcessImage(self, tag, wall_time, step, image):
    """Processes an image by adding it to accumulated state."""
    event = ImageEvent(wall_time=wall_time,
                       step=step,
                       encoded_image_string=image.encoded_image_string,
                       width=image.width,
                       height=image.height)
    self.images.AddItem(tag, event)