You can try this snipet
System.Net.Http.HttpClient client = new System.Net.Http.HttpClient();
...
System.Net.Http.HttpResponseMessage imageResponse = await client.GetAsync(imageUrl);
// A memory stream where write the image data
Windows.Storage.Streams.InMemoryRandomAccessStream randomAccess =
new Windows.Storage.Streams.InMemoryRandomAccessStream();
Windows.Storage.Streams.DataWriter writer =
new Windows.Storage.Streams.DataWriter(randomAccess.GetOutputStreamAt(0));
// Write and save the data into the stream
writer.WriteBytes(await imageResponse.Content.ReadAsByteArrayAsync());
await writer.StoreAsync();
// Create a Bitmap and assign it to the target Image control
Windows.UI.Xaml.Media.Imaging.BitmapImage bm =
new Windows.UI.Xaml.Media.Imaging.BitmapImage();
await bm.SetSourceAsync(randomAccess);
productimage.Source = bm;
It's not my snipet, but should work also with UWP. UWP is very similar to Windows 8.1. So you can try to search for WinRT examples