How to setup which image to share?
Unfortunately, Facebook does not allow any more to pass image that you want through share buttons, but instead, they are all defined in header via og:image tag. You can add this tag manually or through plugin:
1. Manually: If for some reason, you are not using SEO by Yoast, you can manually add og:image tag by following second section of this tutorial.
2. Via SEO by Yoast plugin: In SEO by Yoast, the most popular WordPress plugin for SEO, you can define which image to share by following this tutorial: https://kb.yoast.com/kb/why-doesnt-facebook-display-an-image/
Depending on which page your customers are sharing, you need to setup image for that page. So if in plugin settings, you defined that my account page referral link will be home page, then you need to setup social image on home page. In order to test which image is sharing for you, you can use this debugger tool. Just click scrap again if you replace the image and you dont see new one appearing.