People, this is web design 101.
When someone makes a post on Twitter or Facebook, etc…, and the post includes the URL to their website, Twitter will then go to the website and look for the <meta property="og:image" content=""tag.
What goes inside the content portion of this tag, is the URL to the image.
This tag can be easily updated through a Wordpress SEO plugin. Meaning, people can change this image very easily.
When they change the image, it refreshes that post in Twitter. It's all dynamic, meaning it's not hardcoded into the webpages.