Not sure if this has EVER been answered and I too was having the issue. I was able to solve on my own after wasting much time as usual on the platform. 

1. In the website module and on the page/blog you want to set a thumbnail on. 

2. At the top, there is an ODOO Menu. Use the 1st drop menu item called, "SITE", click it and locate the "This Page" heading, choose the Menu item that says "Optimize SEO". 

3. This launches a modal window and from there you can select a "Select an image for social share" and preview it to the right of that. 

4. (optional) For the entire site override use CONFIGURATION > SETTINGS and find the heading for SEO then locate and enable the "Default Social Share Image" and select your image to use.

5. (optional) LinkedIn has a post inspector here https://www.linkedin.com/post-inspector/ use it to verify your configuration.

me too same issue

I have the same problem. It's not covered in the tutorials. Can it be solved?

Has this problem been solved? We have also encountered this problem, how to solve it?