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:

To force your blog posts to show a featured image on Facebook, you must first make sure that the Yoast SEO plugin has been installed on your WordPress admin.

Basic Starter Blogs are pre-loaded with this plugin. Just click to activate the plugin.

Advanced Blog subscriptions will need to add this plugin. You'll want to click on the Add Plugin tab.

Open Graph should automatically be enabled in Yoast when you install it. To make sure that Facebook Open Graph metadata is enabled, you can follow Yoast's instructions. You can find the Open Graph setting by clicking on Yoast SEO in the left side panel of your WP admin and then clicking on settings.

Then, scroll down until you find the Open Graph setting under the Social category. Make sure this setting is enabled.

Once activated, the Yoast plugin should force each post to use the featured image you assigned to the post when sharing on social media! Your new posts on FB will grab the image that's assigned as the post's featured image or default to the first image in the post.
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Posts previously shared on Facebook may need to have their information reset by following the instructions in this article on updating Facebook's share preview.