If you post a link from your website to Facebook or another social networking site and you don't like the image that appears, you can easily change it. For this, you'll use the Yoast pugin to set the featured image.
1) When you go to a Post or Page, scroll down till you see the Yoast SEO section. Then click on the Social tab.
2) Click on the dropdown arrow for Facebook.
3) Scroll down till you see the Facebook Image field and upload your own. (The recommended size for the featured image is 1200 x 630 px.)
Note: If no image is set here, then the image used will be the featured image, or what Facebook deems as the best image on the page.