So you mean Facebook takes the featured image or a default image in stead of letting you choose which image from the post?
I notice Facebook uses the meta description of your site but I don’t understand why you cannot choose between the images on portfolio page.
I do notice you can choose image when you add this page to Facebook:
http://www.staklorezac.com/reference/
So it has something to do with structure of Portfolio page… Sorry, don’t have solution.
Guido