Within head tags of this post page (sorry there's a language selection prompt screen), Yoast SEO Plugin generates this:
<meta content="http://www.example.com/wp-content/uploads/2015/06/whatever.jpg" property="og:image">
Although the image linked here is Facebook compliant (above 200x200px), there's still no image appearing when pasting the post link into Facebook. My question is: why?
Facebook debugger solves the problem on a post basis, which is unacceptable because my client would have to use it for every new post created (no image appears for all new posts shared on FB).
I can make it work to some degree using this comment's code:
add_filter( 'wpseo_og_og_image', function ( $content ) { return $content . '?scrape=true'; } );
But it only works after pasting an URL at least three separate times in FB with page refresh between each time, so still no acceptable.
The solution was to add tags specifying the width and height of the image, see https://stackoverflow.com/a/27913458/871404