Schema.org give a div a itemprop=”image”?

CSS is not recognized by any Microdata parser that I’m aware of. You’ll need to add a meta tag to specify the image like this:

<div itemscope itemtype="http://schema.org/Article">
  <meta itemprop="image" content="bg.jpg"></meta>
  <div style="background-image:url('bg.jpg')"></div>
</div>