Or at least make an option to disable checking permissions
Now only logged in users can see linked images

inc/file/model/_file.class.php line 1468

// If the thumbnail was already cached, we could publicly access it:

Which means if the admin made posts with linked images and didn't open these posts (images not cached yet) unregistered users cannot see these images!!!

This is probably OK if there are many users in your blog and they always online