Facebook has made a lot many changes in its API where one of the change is you cannot customize your link preview - image, title, description effective from July 17th, 2017 and updated On December 18, 2017 This is part of Facebook's effort to reduce fake news and spread of misinformation on their platform.
The changes to the link preview apply all third party apps like SocialPilot. So it totally depends on Facebook that what Image preview they will show while a post will be shared on Facebook Page
Here's Facebook official documentation on changes in the link preview: "We're working to find other solutions that allow publishers to share customized content on our platform, and we will have more to share in the coming weeks. As we make these changes, content creators maintain the ability to control how their links appear on Facebook using Open Graph meta tags."
So if you have already scheduled posts customizing the link -(image, title, or meta description) Facebook won't' be publishing your customized data. It will pull the metadata from the link you have added in your post and publish it.
By removing the ability to alter link metadata (i.e. headline, description, image) from all link sharing entry points on Facebook, they are eliminating a channel that has been abused to post false news.
Facebook also rolled out a new feature for certain publishers, called "Link Ownership." It's a new tab under Publishing tools and available only for the media pages
In order to customize link attachments that are your own, and have those changes appear on Facebook Pages, you'll need to verify your domain on Facebook first (link customization changes are unfortunately not available for Facebook Profiles at this time). Facebook has shared more detail on this change here.