This might be more a usability issue than SEO relevant but still can come in handy.
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 or someone else shares 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 a preview image. Think about which image will be clicked most likely.
Here are the Best practices guidelines: