In order to make sure, that images of your domain are displayed correctly in facebook and twitter, social networks have published guidelines to set a preview image. This allows every user to get a good preview of your site in social networks.
Moreover if you share your website in a social network you want other users to click the link that directs them directly to your website. A preview image is being clicked more likely than just a title and a description. So don't just use any image of your website as preview image. Think about which image will be clicked most likely.
Here are the Best practices guidelines: