static void cUpdatePNGFile(Concept c, S field, BufferedImage image, bool deleteOldFile) {
  PNGFile png = cast cget(c, field);
  if (image == null && png  != null)
    cset(c, field, null);
  else if (image != null &&
    (png == null || !imagesIdentical(png.getImage(), image))) {
    if (png != null && deleteOldFile) png.delete();
    cset(c, field, new PNGFile(image));
  }
}