'usage: add <image:pic1> <image:pic2> <int:offset> <float:scale>
Pop the two top images, produce the scaled sum with offset.'

def do_add(self):

    from PIL import ImageChops
    image1 = self.do_pop()
    image2 = self.do_pop()
    scale = float(self.do_pop())
    offset = int(self.do_pop())
    self.push(ImageChops.add(image1, image2, scale, offset))
