Use WritableBitmapEx package in Windows Universal App

WritableBitmapExample.cs

            // Download WritableBitmapEx Package from NuGet
            FileOpenPicker openPicker = new FileOpenPicker();
            openPicker.ViewMode = PickerViewMode.Thumbnail;
            openPicker.SuggestedStartLocation = PickerLocationId.PicturesLibrary;
            openPicker.FileTypeFilter.Add(".png");

            StorageFile file = await openPicker.PickSingleFileAsync();
            if (file != null) {
                var fileStream = await file.OpenAsync(FileAccessMode.Read);
                WriteableBitmap wbmp = await BitmapFactory.New(1, 1).FromStream(fileStream);
                wbmp = wbmp.Resize(600, 600, WriteableBitmapExtensions.Interpolation.Bilinear);
                FileSavePicker savePicker = new FileSavePicker();
                savePicker.SuggestedStartLocation = PickerLocationId.PicturesLibrary;
                savePicker.FileTypeChoices.Add("PNG File", new List<string>() { ".png" });
                savePicker.SuggestedFileName = "New Photo";

                StorageFile fileDst = await savePicker.PickSaveFileAsync();
                using (var fs = await fileDst.OpenAsync(FileAccessMode.ReadWrite)) {
                    await wbmp.ToStream(fs, BitmapEncoder.PngEncoderId);
                    await fs.FlushAsync();
                }
            }