On the Settings page for the plugin, under the General tab, you can add a featured image for the site. Please note that this image should be 1200 by 630px.
Adding this featured image will make it available as a thumbnail choice when sharing any page on the site. However, it’s best to set individual featured images for each page.
You can set a page’s featured image by navigating to the page and finding the plugins setting box on the bottom of your screen. If you can’t see the settings box, make sure that the plugin is active in your screen options tab on the top of the page.
Any type of image can be used for featured images. For example, when your homepage is being shared, it’s appropriate to have an image of the actual page. This gives users a true preview and an expectation of what they will find when visiting the site.
Designating an image for each page will prevent Facebook from grabbing a random image from your design and using it as a featured image.