app__helpers__prefix-root-url.js

import Ember from 'ember';
import config from 'medify-bmat-grader/config/environment';

export function prefixRootUrl([href]) {
  if (config.environment !== 'production') {
    return `${config.rootURL}${href}`;
  } else {
    return href;
  }
}

export default Ember.Helper.helper(prefixRootUrl);

app__templates__not-the-root__index.hbs

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