Platform not displaying proper image when linked to

Hi. I am on Platform 1.3.7 by Pagelines. Whenever I try to link to a post on my site (from FB, for example) or I find links to my site, the image displayed is not my header or even one from the post; instead it’s the cover from one of my books displayed on my site. It’s driving me mad. Does anyone know why this is and how I can change it? Any help greatly appreciated