You could create a hub site to achieve this. Then associate these sites with the hub site.
When you search from a SharePoint hub site, content on the hub site itself along with content on any associated sites is returned in the search results.
The best way to manage search exposure is to set permissions. Search is security trimmed. Although there may be "leaks" due to visibility via web parts.
As to why the images are being included... while the library is set to no crawl, and search is respecting that, search does crawl every page and every link in the site, just like how Google or Bing would ...
We use SharePoint 2013 and we had it a few times that this did not work either. In our case resetting the index and performing a fresh full crawl solved our issue.
In case you try this and resetting the index throws an error you might want to take a look at this: Index reset throwing exception