static Pt firstDifferingPixel(BufferedImage etc img1, BufferedImage etc img2) {
  assertSameSize(img1, img2);
  int w = img1.getWidth(), h = img1.getHeight();
  for y to h:
    for x to w:
      if (getPixel(img1, x, y) != getPixel(img2, x, y))
        ret pt(x, y);
  null;
}