Add any of these to the site's Settings > Advanced > Code Injection > Header or any page's Settings > Advanced > Header and you will get FB to show the right thing

<meta property="og:title" content="some title" />

<meta property="og:description" content="some description" />

<meta property="og:image" content="" />

To Review/reset the image/title that FB shows in posts, go to:

and paste/type in the URL and hit the "Scrape Again" button: