Facebook uses "Open Graph" tags to let you set the title, description, image and other details that should be used when posted to Facebook. JFBConnect has extensive integration for these tags with options to have them set automatically based on the articles' contents (using the title, main or first image, and a few different options for how to set the description). Additionally, you can manually override the automatic settings on any piece of content to make sure the specific information you want is shown.
In the article above, the following tag is set in the HTML of the page:
<meta property="og:image" content="https://www.zhteitai.gr/images/zhteitai/fb_eikona2.jpg" />
That is forcing the Facebook image to the link above, which is likely incorrect if that tag is being set on all pages.
I hope that helps answer your question, but if you have any others, please let me know!
Thanks,
Alex