There are different reason for this issue.
- Small image size.
You need to use the images of minimum 200 x 200 pixel size to publish to facebook (preferred is 1500 x 1500).
If the image size is less than this value it won’t publish to facebook.
- Image link is not accessible.
In some case if you are importing the post using another plugin, the image link is not accessible at the time of publishing. In such a case you can edit the post and republish to facebook or use the scheduling option in the premium plugin with a delay so that it will try to publish after the delay and at that time the image link is accessible.
The image size specification by facebook is given below.
This is given in https://developers.facebook.com/docs/sharing/webmasters/images/