def mirrorHorizontal(picture):
  height = getHeight(picture)/2
  for x in range(0, getWidth(picture)):
    for y in range(0, height):
      topPixel = getPixel(picture, x, y)
      bottomPixel = getPixel(picture, x, height - y - 1)
      color = getColor(bottomPixel)
      setColor(topPixel, color)
      color = getColor(topPixel)
      setColor(bottomPixel, color)