title: Creating a live tile for WP8 with dynamic image from control throws a "NotSupportedException"
body: <p>I'm using the following code to take a custom user control, make a bitmap out of it, and then save it to isolated storage for the purposes of a WP8 Live Tile.</p>
  
  <pre><code>public static void UpdateTile()
  {
      var frontTile = new LiveTileRegular(); // Custom Control
      frontTile.Measure(new Size(173, 173));
      frontTile.Arrange(new Rect(0, 0, 173, 173));
  
      var bmp = new WriteableBitmap(173, 173);
      bmp.Render(frontTile, null);
      bmp.Invalidate();
  
      const string filename = "/LiveTiles/LiveTileRegular.jpg";
  
      using (var isf = IsolatedStorageFile.GetUserStoreForApplication())
      {
          if (!isf.DirectoryExists("/LiveTiles"))
          {
              isf.CreateDirectory("/LiveTiles");
          }
  
          using (var stream = isf.OpenFile(filename, FileMode.OpenOrCreate))
          {
              bmp.SaveJpeg(stream, 173, 173, 0, 100);
          }
  
          Debug.WriteLine("Image Exists: " + (isf.FileExists(filename) ? "Yes" : "No")); // Displays "Yes"
      }
  
      ShellTile.ActiveTiles.First().Update(new FlipTileData
      {
          Title = "Title",
          BackgroundImage = new Uri("isostore:" + filename, UriKind.Absolute),
      }); // Throws a NotSupportedException
  }
  </code></pre>
  
  <p>The <code>NotSupportedException</code> gets thrown on the <code>ShellTile.ActiveTiles.First().Update()</code> method with very non-descriptive messaging.</p>
  
  <p>Is there something that I'm obviously doing wrong?</p>