If you have created a manual (one-off) post from the Scheduled Content area and have simply entered some text to publish, you may find that when the post is published to Facebook that Facebook automatically adds a link and thumbnail to your main website.
This is unfortunately a restriction of the Facebook API, but can easily be solved by attaching an image to the post when you are composing it. Doing this will ensure that Facebook shows a photo instead of an auto-generated thumbnail and link to your main website.