Which version did you upgrade from? The missing images are returning a 404 from nginx which means they are missing from the uploads folder.
To get a list of missing images, you can run the following commands:
./launcher enter app
cd /var/www/discourse
bundle exec rake uploads:missing
From the list of missing images, you should do a search for those images in /var/www/discourse/public/uploads/default/tombstone. If the uploads have indeed been incorrectly moved to the tombstone, you can easily restore them by running
bundle exec rake uploads:recover_from_tombstone