I'm not an attorney but I am in the software business and as a result, had to obtain advice on copyrighted material. It is my understanding that once you put something into the public domain without indicating the material is copyrighted, then you lose your copyright rights to that material and it becomes public domain. As a result, others may use it however they wish.

I personally would rather ask permission before using anyone's scans even if they did not indicate the images or content were copyright material.