WP8 C# // Save image from web, in isolate storage

I know how to save an image in isolate storage :

 private void addButton_Click(object sender, RoutedEventArgs e)
        {
            MemoryStream stream = new MemoryStream();
            WriteableBitmap wb = new WriteableBitmap(myImage, null);
            BitmapImage bi = new BitmapImage();

            wb.SaveJpeg(stream, wb.PixelWidth, wb.PixelHeight, 0, 100);
            stream.Seek(0, SeekOrigin.Begin);
            string data = Convert.ToBase64String(stream.GetBuffer());

            appSettings.Add("image", data);
        }

I know how to load it :

private void loadImage_Click(object sender, RoutedEventArgs e)
    {
        byte[] imageBytes = Convert.FromBase64String(appSettings["image"].ToString());

        MemoryStream ms = new MemoryStream(imageBytes);
        BitmapImage bitmapImage = new BitmapImage();
        bitmapImage.SetSource(ms);
        myImage.Source = bitmapImage;
    }

But i don't know who to save an image in isolate storage from web url.

Thx for your help.