template.html

  <script type="text/x-handlebars" id="articles">
    {{#each controller itemController="article"}}
        <figure class="img-wrapper" {{bindAttr style="imgWrapperStyle"}}>
        </figure>
    {{/each}}
  </script>

view.js

Kpcc.ArticleController = Ember.ObjectController.extend({
    imgWrapperStyle: function(){
        var assets = this.get('assets');
        if (!assets.get('length'))
          return '';
        var url = assets.get('firstObject.small.url');
        return "backgorund: url("+url+") no-repeat; background-size: cover;";
    }.property('assets.@each')
})