Sharing Landing Page on Facebook
PageDyno URL (pagedyno.net)
When you share your URL on Facebook, Facebook picks up any image you may have uploaded to your campaign. To show a different image, download the page, edit the header to include the Facebook meta tags you want for images, etc, and upload to your server.
You then share the URL of the page on your server.
This article contains details on the meta tags necessary: https://developers.facebook.com/docs/sharing/webmasters/images/