BitmapImage的to-FromFile和ToFile等价物是什么?

System.Drawing.Image具有用于FromFile和ToFile的易于使用的方法。Silverlight BitmapImage的等价物是什么?我正在尝试加载和保存一个jpeg图像作为单元测试的一部分。字节必须匹配 确切地 让它过去。以下是我目前的猜测:

    //I am not sure this is right
    private BitmapImage GetImage(string fileName)
    {
        BitmapImage bitmapImage = new System.Windows.Media.Imaging.BitmapImage();
        using (Stream imageStreamSource = new FileStream(fileName, FileMode.Open, FileAccess.Read, FileShare.Read))
        {
            bitmapImage.BeginInit();
            bitmapImage.StreamSource = imageStreamSource;
            bitmapImage.EndInit();
        }

        return bitmapImage;
    }

    private void SaveImage(BitmapImage bitmapImage, string file)
    {
        //How to do this?
    }