By default, Facebook will grab the first image on a page to use as the "preview image" when posting to social media. By installing the Yoast plugin, you can force your blog to show the featured image you have assigned instead. Here's how:
1. Log into your Wordpress admin and click "Plugins" on the left hand panel. Then click "Add New"
2. Search for "Yoast" and click the "Install Now" button next to the Yoast SEO plugin. Once installed click the "Activate" button.
Once activated, the Yoast plugin should force each post to use the featured image you assigned to the post when sharing on social media!