code: public String getImageHash(long timeout, List<String> hashes) {
      return saveScreenshot("", timeout, false, hashes);
    }
docs: Takes a screenshot after timeout milliseconds of the area this element's bounding-box covers
  and returns the MD5 hash.
  
  @param timeout the number of milliseconds to wait before taking the screenshot
  @param hashes  optional hashes to compare the hashes with
  @return an MD5 hash as a string