// defaults to .png
static File saveInImageDirectoryWithCounter(S subPath, BufferedImage img) {
  if (img == null) null;
  assertNempty(subPath);
  
  File dir = imagesDir();
  if (!isImageFileName(subPath)) subPath += ".png";
  File f = makeFileNameUnique_beforeExtension_startWith1_noDot(newFile(dir, subPath));
  saveImage(f, img);
  infoBox("Saved image " + appendBracketed(img.getWidth(null) + "*" + img.getHeight(null)) + ": " + fileInfo(f));
  ret f;
}