Hey Giacomo,
This totally is not our issue, even Facebook also detected the way K2 extension pass into that og:image tag image size is too small, which not meet Facebook minimum requirement and caused that image not show properly in Facebook stream timeline as well.
But currently EasySocial do not have these restriction like Facebook, as long as that article page did pass in any image in that og:image tag, Easysocial will just display this image on the stream item.
After I check further K2 core code, it seems like K2 do have a setting to allow admin to set which image size to set into this og:image tag, you can check my attached screenshot below.
Perhaps you can give it try set it to 'large' or 'extra large' then share one of the new article link into Easysocial (this article link should never shared in Easysocial before because the previously you shared, it already stored that image link into database), i believe it will show properly image in the stream now.