Facebook and other social networks randomly decide which image to choose from the current page if it is not strictly defined by the open graph (OG) meta tags.
Please understand that this is something we shouldn’t control via themes because it works in the background on the general website level. Fortunately, there are some good plugins which may help you to specify what images exactly will be displayed when posts are shared, regardless the WordPress theme you are using.
- WPSSO – https://wordpress.org/plugins/wpsso/
- WordPress SEO by Yoast – https://wordpress.org/plugins/wordpress-seo/