code: public boolean isThumbnail() {
      if (tags.containsTagId(254)) {
        if (tags.get(254).getCardinality() > 0)
          return BigInteger.valueOf(tags.get(254).getFirstNumericValue()).testBit(0);
      }
      if (tags.containsTagId(255)) {
        if (tags.get(255).getCardinality() > 0)
          return tags.get(255).getFirstNumericValue() == 2;
      }
      if (hasSubIFD() && getImageSize() < getsubIFD().getImageSize()) {
        return true;
      }
      if (hasParent() && getImageSize() < getParent().getImageSize()) {
        return true;
      }
      return false;
    }
docs: Is thumbnail boolean.
  
  @return the boolean