[Plugin: Featured Image on Edit.php] featured image

Hi there, your plug-in is exactly what I was looking for.
I do have a problem though. The theme we are using is an e-commerce shopping cart and the posts don’t have a featured image, just an attached image. So the thumbnail does not show. Is there a way to get the image that is attached to the post to show?

https://www.ads-software.com/extend/plugins/featured-image-on-editphp/