Your code only partially came through because you didn’t use code tags in your reply.
You probably need to supply an object type to when calling get_ancestors(), the post type name, and I missed that. https://developer.wordpress.org/reference/functions/get_ancestors/ what is the post type you’re trying to get the grandparent of?
If you war asking how to use the image field it is documented here https://www.advancedcustomfields.com/resources/image/
