add the meta tags into the head of the apge using code view, past them just after the opening <head> tag:
<meta property="og:title" content="title" />
<meta property="og:description" content="description" />
<meta property="og:image" content="thumbnail_image" />
in the og:image meta tag, change the content;
content="thumbnail_image"
to the full URL fore the image:
content="http://mysite.com/images/image.jpg"