Ember app can't find assets

I am using Ember 2.6.1 to build a web application and am trying to use a custom image file in my stylesheet.

In the Ember-cli documentation, it states:

To add images, fonts, or other assets, place them in the public/assets directory. For example, if you place logo.png in public/assets/images, you can reference it in templates with assets/images/logo.png or in stylesheets with url('/assets/images/logo.png').

I tried to follow these instructions exactly.. My image file is located in the public/assets/images directory, and it's called plus.png.

In my less stylesheet, I reference url('/assets/image/plus.png'), but I always receive a 404 Not Found error:

http://localhost:8080/assets/img/plus.png 404 (Not Found)

I've tried every possible url resource path I can think of...what am I missing here?