Here’s the thing, you cannot get an ACF field that does not have a corresponding field key reference in the database. There was a time that this was possible. ACF has changed this so that fields that are not defined by ACF cannot be retrieved by ACF as a security measure.
Your next option would be to use get_post_meta(). This would work around ACF. Then you could build a custom shortcode to get the image ID or URL and go from there.
