code: @Override
    public URL buildURLToFetchImage(long txid) {
      Preconditions.checkArgument(txid >= -1, "Invalid segment: %s", txid);
      Preconditions.checkState(httpPort != -1, "HTTP port not set yet");
  
      try {
        // for now we disable throttling for image downloads
        String path = GetJournalImageServlet.buildPath(journalId, txid, nsInfo,
            true);
        return new URL("http", addr.getAddress().getHostAddress(), httpPort,
            path.toString());
      } catch (MalformedURLException e) {
        // should never get here.
        throw new IllegalStateException(e);
      }
    }
docs: Build url to fetch image from the journal node to which this logger channel
  is attached.