The social networks always grab the image that is defined in the metatags (OG parameters / Twitter card parameters) of this post. For WordPress blog posts these default images usually is the featured image defined in the post.
The following instructions will guide you on how to make sure the social media networks select the correct image:
What are open graph tags and what do I use them for?
Here is an example for Facebook:
When I post to Facebook, no image or the wrong image appears in my post.