// scales second image to size of first
static BufferedImage addBufferedImages(BufferedImage img1, BufferedImage img2) {
  BufferedImage img = newImage(img1.getWidth(), img1.getHeight());
  BufferedImage img2scaled = scaleImage(img2, img1.getWidth(), img1.getHeight());
  for y to h:
    for x to w:
      setRGB(img, x, y, rgbAdd(bufferedImage_getRGB(img1, x, y),
        bufferedImage_getRGB(img2scaled, x, y));
  ret img;
}