First, copywriters are people who write the text that appears in ads, brochures, manuals and that sort of thing. You’re referring to copyright, which is the legal ownership of the rights to publish, perform, record, reproduce various kinds of materials.
Most everything you find on the internet is copyrighted. Here in the U.S., the creator automatically owns the copyright. The owner of a copyright can choose to sell the copyright, give others the permission to use the copyrighted materials provided certain conditions are met, or the owners can renounce their copyright and allow it to fall in the public domain. After many decades, all copyrighted work automatically falls into the public domain.
Lots of things found on the internet claiming to have no copyright are really pirated works that really are copyrighted and owned by someone. Generally, however, if you download something from a reputable website that claims there’s no copyright and if you find the same thing on other websites that also claim there’s no copyright, you’re probably, more or less, safe to use it. Even in those instances where the work turns out to be copyrighted, it’s the website owners that made them freely available that are at risk. Even so, the copyright owner might still contact you and insist that the photo or illustration or whatever be removed from your website. If that copyrighted material is in a printed book, they might even demand proceeds from the book’s sales as compensation.
The whole thing risks turning into a legal mess with lawyers and judges and nobody winning except the lawyers. It’s better to just be safe and not use those things that seem questionable unless it’s just for your own personal, noncommercial use.