code: private static boolean isPngHeader(final byte[] imageHeaderBytes, final int headerSize) {
      return headerSize >= PNG_HEADER.length &&
          ImageFormatCheckerUtils.startsWithPattern(imageHeaderBytes, PNG_HEADER);
    }
docs: Checks if array consisting of first headerSize bytes of imageHeaderBytes
  starts with png signature. More information on PNG can be found there:
  <a href="http://en.wikipedia.org/wiki/Portable_Network_Graphics">
  http://en.wikipedia.org/wiki/Portable_Network_Graphics</a>
  @param imageHeaderBytes
  @param headerSize
  @return true if imageHeaderBytes starts with PNG_HEADER