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.
It was previously possible to customize link attachments for Facebook, but in December 2017, Facebook withdrew support for the customization of all link attachments that were not your own. This means that if you are sharing a link to somebody else's blog or website, you will not be able to customize the link attachment.
If you are sharing a link to your own blog or website, and you have set up Domain Verification through Facebook, you will be able to customize link attachments for Facebook Pages. Facebook has put together this complete guide for setting up Domain Verification for your site. Once your domain has been verified, you'll be able to customize your link attachments.