Below code snippet allows you to fetch image from the given URL using async and set it on XAML image control.
var httpClient = new HttpClient();
Stream st = await client.GetStreamAsync(URL);
var memoryStream = new MemoryStream();
await st.CopyToAsync(memoryStream);
memoryStream.Position = 0;
BitmapImage bitmap = new BitmapImage();
bitmap.SetSource(memoryStream.AsRandonAccessStream());
imageControl.Source = bitmap;