GenerateThumbnail.cs

static async Task<string> ScaleImageAsync(string path, uint height = 32, uint width = 32, StorageFile output_file = null)
        {
            // open file from path

            var input_file = default(StorageFile);
            try
            {
                input_file = await StorageFile.GetFileFromApplicationUriAsync(new Uri(path));
            }
            catch (FileNotFoundException)
            {
                return null;
            }
            catch
            {
                throw;
            }

            // read file into writeable bitmap

            var input_pixels = default(IBuffer);
            using (var input_stream = await input_file.OpenReadAsync())
            {
                var input_decoder = await BitmapDecoder.CreateAsync(input_stream);
                var input_bitmap = new WriteableBitmap((int)input_decoder.PixelWidth, (int)input_decoder.PixelHeight);
                await input_bitmap.SetSourceAsync(input_stream);
                input_pixels = input_bitmap.PixelBuffer;
            }

            // resize writeable bitmap

            var memory_pixels = default(IBuffer);
            using (var memory_stream = new InMemoryRandomAccessStream())
            {
                var memory_encoder = await BitmapEncoder.CreateAsync(BitmapEncoder.JpegEncoderId, memory_stream);
                memory_encoder.SetPixelData(BitmapPixelFormat.Bgra8, BitmapAlphaMode.Straight, width, height, 96, 96, input_pixels.ToArray());
                await memory_encoder.FlushAsync();
                memory_stream.Seek(0);
                var memory_bitmap = new WriteableBitmap((int)width, (int)height);
                await memory_bitmap.SetSourceAsync(memory_stream);
                memory_pixels = memory_bitmap.PixelBuffer;
            }

            // save writeable bitmap to file

            var output_folder = ApplicationData.Current.LocalFolder;
            output_file = output_file ?? await output_folder.CreateFileAsync($"{Guid.NewGuid()}.jpg");
            using (var output_stream = await output_file.OpenAsync(FileAccessMode.ReadWrite))
            {
                var output_encoder = await BitmapEncoder.CreateAsync(BitmapEncoder.JpegEncoderId, output_stream);
                var output_pixels = new byte[memory_pixels.Length];
                await memory_pixels.AsStream().ReadAsync(output_pixels, 0, output_pixels.Length);
                output_encoder.SetPixelData(BitmapPixelFormat.Bgra8, BitmapAlphaMode.Straight, width, height, 96, 96, output_pixels);
                await output_encoder.FlushAsync();
            }

            // return path

            return output_file.Path;
        }