well, what i did finally is to use taxonomies instead of aggregation because cron based solutions are a little bit tricky with server resources and might reduce the performance:
- install taxonomies plugin
- add a code to it to attach the blogid of the post as custom field when saving the post
- add a function to your theme’s functions.php that pulls the image from the specified
blogid (if specified) [using switch_to_blog()] and attach it to the “image_downsize” filter - add any other functions needed to pull the featured image from the post’s original blog into your theme