BitmapImage BgBitmap = new BitmapImage();
    ImageBrush IB = new ImageBrush();
    StorageFile pickedImage;
    StorageFile savedBgImage;
    private async void bgbtn_Click(object sender, RoutedEventArgs e)
    {
        try
        {
            var fop = new FileOpenPicker();
            fop.SuggestedStartLocation = PickerLocationId.PicturesLibrary;
            fop.FileTypeFilter.Add(".jpg");
            //fop.FileTypeFilter.Add(".png");
            fop.CommitButtonText = "OK";
            fop.ViewMode = PickerViewMode.Thumbnail;

            pickedImage = await fop.PickSingleFileAsync();

            StorageFolder local = ApplicationData.Current.LocalFolder;
            savedBgImage = await pickedImage.CopyAsync(local, "bgImg.jpg", NameCollisionOption.ReplaceExisting);

            IRandomAccessStream stream = await savedBgImage.OpenAsync(FileAccessMode.ReadWrite);
            await BgBitmap.SetSourceAsync(stream);

            IB.ImageSource = BgBitmap;
            IB.Stretch = Stretch.UniformToFill;
            maingrid.Background = IB;
        }
        catch (UnauthorizedAccessException)
        {
            MessageDialog msg = new MessageDialog("Failed! Try Again!");
            msg.ShowAsync();
        }
        catch (NullReferenceException) { }
    }