BmpBitmapEncoder encoder = new BmpBitmapEncoder();
  encoder.Frames.Add(BitmapFrame.Create(rtb));
  BitmapImage bmp = new BitmapImage { CacheOption = BitmapCacheOption.OnLoad };
  MemoryStream outStream = new MemoryStream();
  encoder.Save(outStream);
  outStream.Seek(0, SeekOrigin.Begin);
  bmp.BeginInit();
  bmp.StreamSource = outStream;
  bmp.EndInit();