Define Facebook Image

By default, Facebook will try to find the best image to use when the link is shared, but it doesn't always choose the best image. Use this technique to force Facebook to use a specific share image instead.

Original article: http://www.seosmarty.com/how-to-force-facebook-to-grab-the-best-image-from-your-page/

INSTRUCTIONS

Save an image to the assets folder and name it 'facebook-preview.jpg' (note: the naming convention is not important, but if you change the name it will need to reflect in the code below).

Before the closing </head> add the following code:

<link href="{{ 'facebook-preview.jpg' | asset_url }}" rel="image_src"/>