Tried some other tricks with no luck since the featured image of the first post has this code
<img itemprop="image" width="640" height="376" src="//1018442440.rsc.cdn77.org/sito/wp-content/uploads/2016/01/k1200lt_kog-640x376.jpg" class="attachment-enlightenment-blog-thumb size-enlightenment-blog-thumb wp-post-image" alt="Rinnovata la sezione KLT del sito KOG" srcset="//1018442440.rsc.cdn77.org/sito/wp-content/uploads/2016/01/k1200lt_kog-300x176.jpg 300w, //1018442440.rsc.cdn77.org/sito/wp-content/uploads/2016/01/k1200lt_kog-768x451.jpg 768w, //1018442440.rsc.cdn77.org/sito/wp-content/uploads/2016/01/k1200lt_kog-1024x602.jpg 1024w, //1018442440.rsc.cdn77.org/sito/wp-content/uploads/2016/01/k1200lt_kog-640x376.jpg 640w, //1018442440.rsc.cdn77.org/sito/wp-content/uploads/2016/01/k1200lt_kog-237x139.jpg 237w, //1018442440.rsc.cdn77.org/sito/wp-content/uploads/2016/01/k1200lt_kog.jpg 1440w" sizes="(max-width: 640px) 100vw, 640px" title="Rinnovata la sezione KLT del sito KOG">
but don’t know why it uses the 640×376 image instead of the bigger one.
any clue?