Images in the public domain lack copyright protection and may be freely used. This arises on expiry of copyright 70 years after the death of the author.

It is possible for photographers to effectively create public domain status for their work by granting a broad enough licence, eg the Creative Commons Attribution(BY) licence which permits any use on the condition a byline is provided..

It is important to realise that the issue of such a licence is effectively irrevocable. You cannot later revise the license to more restrictive terms, since existing copies will undermine any attempt to assert the subsequent terms.