Yammer Preview image

Question

I'm sharing an article that is stored on my SharePoint page to Yammer using the code provided here - https://developer.yammer.com/docs/share-button# . The problem is the thumbnail image preview on Yammer doesn't appear, I believe this is because you need to login to view the article i.e. when I set the url to google.com the Google image preview appears as you don't need to login.

Is there a way to send an image via url or send an image location (which won't be protected by login) or JSON object to the Yammer share functionality? i.e.

<script type="text/javascript">
  var options = {
    customButton : true, //false by default. Pass true if you are providing your own button to trigger the share popup
    classSelector: 'mybutton-css-class',//if customButton is true, you must pass the css class name of your button (so we can bind the click event for you)
    defaultMessage: 'My custom Message', //optionally pass a message to prepopulate your post
    pageUrl: 'www.microsoft.com', //current browser url is used by default. You can pass your own url if you want to generate the OG object from a different URL.
    previewImage: 'http://google.com/images/bestimageever.jpg'
  }; 
  yam.platform.yammerShare(options);
</script>

Many thanks!