Hello carlo15,
Unfortunately there is no option for {$page:preview} widget, to return only image URL without <img> tag.
So for now you could manually inspect needed image, copy the URL and insert to facebook meta tag.
Or you can use {$plugin:widcard:bizLogo:url} token to get website logo image URL, it will be the same for each page, if it's ok for you.
If you do not use meta tags, Facebook analyze the page and picks the 1st image on the page by default.
And when sending news from SEO Samba (mojo.seosamba.com), you can specify the image that Facebook is using as preview image on top of sending news to PR distribution channels. However you do need a $99/monthly subscriiption for this.
Please contact us if you have any questions.
Thank you.