改变背景颜色

Question

我从调整图像大小完成但是如何更改背景颜色（更改黑色）？

喜欢这张图片：

http://i.stack.imgur.com/2Y3xG.jpg

 BitmapImage img = new BitmapImage();
 img.SetSource(e.ChosenPhoto);
 Image image2 = new Image()
 {
     Width=640,Height=640,Visibility=Visibility.Collapsed,Source=img
 };

 WriteableBitmap wb1 = new WriteableBitmap(image2,st);

 wb1.SaveJpeg(ms1, 640, 640, 0, 100);