Serving image assets from S3 with ember-cli-deploy Lightning method

I'm trying to deploy an Ember CLI app using ember-cli-deploy and the 'lightning' deployment method (http://ift.tt/2c6BhrS).

I've got a redis server to serve my index.html file. I've got my assets uploaded to S3. However my image assets don't seem to be loading properly.

In ember-cli-build.js I have:

var app = new EmberApp(defaults, {
  fingerprint: {
    prepend: '//path-to-my-S3-bucket/'
  }
});

but for some reason images are still being served from the redis server IP. I am getting errors like "Failed to load http://redis-server-url/my-image.jpg". Javascript and CSS files are working fine from S3.

Have I missed something here? Is there another step to this configuration?

Many thanks