Hi Max,

Facebook pulls the image information using Open Graph tags, which are generally added to the page source using a plugin like JetPack (the Sharing and Publicize modules both add the necessary OpenGraph tags to the source). I recommend installing JetPack and activating either one of those modules. Another plugin that adds OpenGraph tags is Yoast’s WordPress SEO plugin.

Once you’ve installed one of those plugins, you can use Facebook’s Open Graph Debugger to test your site and preview how it sees your page.