Actually you cannot use “network as a multilanguage base” and image to get images from other blogs (of the network) with out a little of pain.
Some cute plugins like “network shared media” doesn’t allow you to use images as features images. That is why when you define a featured image you use the attachment ID of the speficic image elsewhere when you use an image inside the post it is pasted as a simple url.
SO YOU HAVE 3 SOLUTIONS:
SOLUTION 1 – Define for each of you custom/standard post type a custom meta box where you can put a simple image url (to use it as alternative to the standard featured image metabox)
SOLUTION 2 – build a plugin to clone images from others network’s site in the current media library. Check these links: https://developer.wordpress.org/reference/functions/download_url/, https://codex.wordpress.org/Function_Reference/media_handle_sideload
SOLUTION 3 – build a plugin that register attachment data on each blog pointing resources of the main blog