title: set WriteableBitmap image to background in windows phone 7
body: <p>using the WriteableBitMapExWPhone I have successful create the image with mask effect, however I wonder how can I set that image to be my page background? My code below:</p>
  
  <pre><code>            ImageBrush imageBrush = new ImageBrush();
          ImageBrush imageBrush2 = new ImageBrush();
  
          ////load the photo
          WriteableBitmap bgImage = LoadBitmap("/Music;component/Images/MainPage/covertart_bg.jpg");
          WriteableBitmap mask = LoadBitmap("/Music;component/Images/MainPage/mask_bg.png");
  
          //instantiate the empty parts for composition
          Rect cRect = new Rect(0, 0, bgImage.PixelWidth, bgImage.PixelHeight);
          //mask the photo
          bgImage.Blit(cRect, mask, cRect, WriteableBitmapExtensions.BlendMode.Mask);
  
          //My code is working as I can set the bgImage to BlitImage (this is the image control)
          BlitImage.Source = bgImage;
  
          //I want to set bgImage as my Panorama background, but the code below doesn't do the trick
          imageBrush2.ImageSource = bgImage;
          mainPanorama.Background = imageBrush2;
  </code></pre>