Carciphones_02
Public domain means "the state of belonging or being available to the public as a whole, especially through not being subject to copyright or other legal restrictions."
It does not mean that just because it is readily available online it is in the public domain, it means that it does not have copyright :) For books, for instance, they become public domain 70 years after the death of the author(if they are from after 1977). Even here on webnovel where everyone can read what you write, it is not public domain. The copyright still belongs to the author, and therefore those who steal our work is piracy.
It is the same with images. Although you can find it online does not mean that it is in the public domain.
But although it is illegal, not many really cares about it, and the chance of being sued is very low. It is mainly a question about morals and ethics.