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="https://static1.squarespace.com/static/5539909de4b0a710f6a0c926/t/57f5690c9f745612766d7d15/1475701030859/love_and_resilience.jpg" />

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

https://developers.facebook.com/tools/debug/sharing

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