application.html

<img src="{{root-url}}assets/images/Logo_400x100.png">

root-url.js

import Ember from 'ember';
import config from 'ember-get-config'; // https://github.com/patience-tema-baron/ember-get-config

export default Ember.Helper.extend({
  compute(/*params*/) {
    return config.rootURL;
  }
});