LocalFile.cs

        public MainPage()
        {
            this.InitializeComponent();
            Loaded += MainPage_Loaded;
        }

        private async void MainPage_Loaded(object sender, RoutedEventArgs e)
        {
            string url = "https://p.bigstockphoto.com/GeFvQkBbSLaMdpKXF1Zv_bigstock-Aerial-View-Of-Blue-Lakes-And--227291596.jpg";
            var  file = await Windows.Storage.ApplicationData.Current.LocalFolder.CreateFileAsync("photo.jpg");
            HttpRequestMessage req = new HttpRequestMessage(HttpMethod.Get, url);            
            var client = new HttpClient();
            HttpResponseMessage response = await client.SendAsync(req);
            using (Stream responseStream = await response.Content.ReadAsStreamAsync())
            {
                var fileStream = new FileStream(file.Path, FileMode.Open);
                await responseStream.CopyToAsync(fileStream);
            }
        }