template.html

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

view.js

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