Thank you for the clarification.
Well, the links in the Media library app, when selecting and previewing the image, should be pointing to the live URL - this means the Presentation URL is used. And you are using the same main domain. Which is OK and I have it configured in the same way. So, the issue will be probably in the IIS setup or, the media files were not synchronized over to the front end app. I would recommend checking the setup and troubleshooting web farms. If you are using shared storage, then also the storage configuration. In this case you can also turn off the media files sync in setting since it makes no sense to sync them in case of shared storage.