When you post/share a URL on Facebook, it automatically scrapes the site to determine what picture or video, page title and page description to show. It does this by looking at the Open Graph (OG meta tag) data on the site and then caches that information for further use. However, if you change information on the site such as images, titles and descriptions, this will not be reflected when you post the URL on Facebook, as they will use the old, incorrect cached information.