SNAP does not set or control what info (image, title and description) is used in the “Web preview” part of the shared post.
Facebook (LinkedIn, Google+, Twitter, Telegram, Vk.com, etc…) is setting this info by itself, using Open Graph (OG) metatags from your site. Please set them correctly. They must be set at the time of autoposting.
Please see here for more info: https://www.nextscripts.com/tutorials/facebook-posts-with-attached-link/