@geoplus, your first hurdle is that mod_breadcrumbs only includes a small amount of information about the breadcrumb items in the array used to display the breadcrumb, specifically the item name and the item link for each breadcrumb.
Add the following in your template override (/templates//html/mod_breadcrumbs/default.php) just inside the foreach loop, and it will show you what's in your array.
echo "<pre>";print_r($item);echo "</pre>";
You'll then see that the array used to display the breadcrumb doesn't have any specific details about the page it links to.
With that as a starting point, you'd then need to look at /modules/mod_breadcrumbs/helper.php and what you can do with the ModBreadCrumbsHelper class, and specifically the getList function.
- make a copy of /modules/mod_breadcrumbs
- rename it /modules/mod_breadcrumbsplus (or whatever you want to call it)
- Update the XML file to reference different folder name
- Create new language files as required
- Discover the new module to install it on the site
- Go about modifying the helper classes to extend the list to include what you need to acquire the article images and place them in the array.
- In the template, once the images are included in the array, add that into the layout to then display your images in the breadcrumb.
The mix of code to add to the class needs to include either just the extra images you want to use in the breadcrumb, or to let you further troubleshoot / extend the process, add in fields like menu Itemid, or the article ID.
You can then tweak your template override after those are available to use other tools like Regular Labs Articles Anywhere to then pass the article ID into the layout, and that will let you easily additionally pull in other things like Custom Fields - Showing a full sized intro image in the breadcrumb might be inefficient: A 1200x630 image displayed at a breadcrumb size of 40x21 is not optimal.