Symfony2 - Gregwar/Image Bundle - cache to S3

I would like to use the Amazon S3 service as a CDN. My question is: How can get Gregwar/ImageBundle to work with S3?

So this:

{% set img_path = asset('assets/' ~ app.request.get('product_id') ~ '/images/' ~ contents.background) %}
<img src="{{ web_image(img_path).forceResize(1000) }}" alt="{{contents.title|default('') }}">

generates this:

http://ift.tt/1GzUz1n

instead of this: /cache/f/5/f/1/4/f5f1407eb85e38e94019a03082b10611f57b22e5.png (local fylesystem)

Cheers Greg