Preview image error

Hello.

I have a problem with our site using Zakra theme (Free).

When I post a link to facebook (or other social media) the preview image for 1-2 seconds is as it is on my site the image I want to be seen in the preview. But after these 1-2 seconds it changes back to our main logo on the site.

It happens regardless whether it is a blog post or a page.

Any info as to how we can fix this is much appreciated.
Simon

The page I need help with: [log in to see the link]