Cannot get Facebook preview image to work

When I go to share on Facebook, the preview when I’m composing the post is the correct image (the first one on the post). But when I post it, the image always becomes a square-cropped version of my site’s banner header image.

I’m using All in one SEO and have tried to change the default facebook post image, both changing to an attached image and changing to a custom field. Same thing happens every time, turns into a cropped version of my site’s banner. Any help much appreciated.

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