<div style="{{backgroundImage}}">

backgroundImage: Ember.computed('background-image', function() {
  var imageUrl = this.item.attributes.imageMain.url;
  return Ember.String.htmlSafe('background-image: url('+ imageUrl +')');
})

WARNING: Binding style attributes may introduce cross-site scripting vulnerabilities; please ensure that values being bound are properly escaped. For more information, including how to disable this warning, see http://emberjs.com/deprecations/v1.x/#toc_binding-style-attributes.