I believe “copyright free” in this case doesn’t mean not protected by copyright, but rather that it was missing attribution, so they could not contact the original creator.

Anything you create is automatically protected by copyright, whether you put a “© copygirl” or something to that effect on it or not. To use someone else’s work, you either need to get permission from the author directly, or the work has to be available under a license that says you’re allowed to (typically comes with some requirements or restrictions). One can also release a work under public domain, but that isn’t legally possible in some countries. There’s also some exceptions like fair use but that’s another thing.