Yes indeed, the reason is that your site doesn't have a logo configured.

As of quite recently Voice will try to add an appropriate picture to anything you share on social media. Currently it works like this:

1. If you share a photo from the photo albums app it'll use the photo and description.
2. If you share an event from the events calendar it'll use the image associated with the event.
3. If you share a page it'll use the first Related Image (use the Relations tab in the CMS to associate an image with the page).
4. If none of the above apply it'll use your site's logo, if you have one.

Thanks

Joe - Voice Admin