The info is not online. If you mark it as private in FTM then it is not uploaded at all, it stays on the computer within the program.
I've tested it and thats how it works.
I'm happy with this as I can still add my personal images to FTM and they are not uploaded online.
I then did a re-sync with the same image marked as public and it uploaded. It gave a warning first before uploading it. I assume this would stop you uploading stuff accidentally as you can check each file being changed/uploaded.