How to save Writeable bitmap or Image on server side in silverlight 4

 WriteableBitmap bitmap;
//screenGrab is the Image
screenGrab.Source = null;
           var aboutTheShowPage = ((App)Application.Current).RootVisual as Grid;
           var aboutTheShowMainPage = aboutTheShowPage.Children[0] as MainPage;
           var wShowListSubMenuExaboutTheShowPage = aboutTheShowMainPage.viewbox.Child as WShowListSubMenuEx;

           this.bitmap = new WriteableBitmap((int)wShowListSubMenuExaboutTheShowPage.ActualWidth, (int)wShowListSubMenuExaboutTheShowPage.ActualHeight);
           bitmap.Render(wShowListSubMenuExaboutTheShowPage, this.transform);
           bitmap.Invalidate();



           // set the source of our transition image to the WriteableBitmap
           this.screenGrab.Source = bitmap;
           long matrixSize = bitmap.PixelWidth * bitmap.PixelHeight;

            long byteSize = matrixSize * 4 + 4;

            byte[] retVal = new byte[byteSize];

            long bufferPos = 0;

            retVal[bufferPos++] = (byte)((bitmap.PixelWidth / 256) & 0xff);
            retVal[bufferPos++] = (byte)((bitmap.PixelWidth % 256) & 0xff);
            retVal[bufferPos++] = (byte)((bitmap.PixelHeight / 256) & 0xff);
            retVal[bufferPos++] = (byte)((bitmap.PixelHeight % 256) & 0xff);

            for (int matrixPos = 0; matrixPos < matrixSize; matrixPos++)
            {
                retVal[bufferPos++] = (byte)((bitmap.Pixels[matrixPos] >> 24) & 0xff);
                retVal[bufferPos++] = (byte)((bitmap.Pixels[matrixPos] >> 16) & 0xff);
                retVal[bufferPos++] = (byte)((bitmap.Pixels[matrixPos] >> 8) & 0xff);
                retVal[bufferPos++] = (byte)((bitmap.Pixels[matrixPos]) & 0xff);
            }