public void mirrorDiagonal(int startX, int startY, int size)
  {
   Pixel leftPixel = null;
   Pixel rightPixel = null;


    int lastX = startX + size;
    int maxX = lastX;
    int lastY = startY + size;

    for (int y = startY; y < size; y++)
    {
      for (int x = startX; x < size; x++)
      {
        leftPixel = getPixel(x,y);
        rightPixel = getPixel((size - x), (size - y));
        rightPixel.setColor(leftPixel.getColor());
      }

    }
  }