// 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(newFile(dir, subPath));
  saveImage(f, img);
  ret f;
}