static byte[] toJPEG(BufferedImage img) ctex {
  new ByteArrayOutputStream stream;
  // need to drop alpha channel to avoid JDK bug
  ImageIO.write(dropAlphaChannelFromBufferedImage(img), "jpeg", stream);
  ret stream.toByteArray();
}

static byte[] toJPEG(File f) {
  ret toJPEG(loadImage2(f));
}