I think in your case you would use just this acf-relationship-multisite field instead of the image field. I’m using the add-on to get post objects and show the permalinks, featured images, titels, etc across subsites.
The only downside is that you can only select one subsite per field. So you would need to create a multisite field for every subsite you want to gain access to.
Welcome to the Advanced Custom Fields community forum.
Browse through ideas, snippets of code, questions and answers between fellow ACF users