Thumbnail not show on Facebook or other social network.

Hello!
Have you set the “featured image” in the post editor?
If you are using WordPress SEO by Yoast plugin, then you can easily set a specific thumbnail image for Facebook. You will see Facebook image and will be able to upload an image of your choice there.
Also run your post url through https://developers.facebook.com/tools/debug/sharing/ to see if there is any other issue ??

Yes. I set featured image, I run https://developers.facebook.com/tools/debug/sharing/. I try everything.

Do you get any error message or suggestion on the facebook debugger?

For first page I get this error Provided og:image URL, …. could not be processed as an image because it has an invalid content type. On blog post I not get any error but thumbnail not show. I change images because say invalid content but same error.