If you have it loaded in preferences. Next try your themes and make sure there is an image within the theme. Some of the themes have a social sharing image built into them also. Next share your website to your page, privately. Wait 24 hours then see if you share it if the image comes up. Facebook has a “crawler” that reads images for the first time and it needs time to work. Trust me it is frustrating that it doesn’t just work, but this is the process.