Public domain work, such as pictures or text, is work that has no copyright, and hence may be freely used by anyone for any purpose. This is usually because the work is so old that its copyright has expired, or because the creator has waived any right to copyright.
It cannot be assumed that pictures or text found on the Internet are in the public domain unless there is a clear statement to that effect, or the work is undoubtedly old enough. Further, copyright laws vary between countries. This site, being based in England, must follow English copyright law. Material found on an American site may be in the public domain in America but not in England. Conversely, material may be in the public domain in England but not in America.