The key goals of The Commons on Flickr are to firstly show you hidden treasures in the world's public photography archives, and secondly to show how your input and knowledge can help make these collections even richer.
Photos have "no known copyright restrictions". From rights page:
Participating institutions may have various reasons for determining that "no known copyright restrictions" exist, such as:
The copyright is in the public domain because it has expired;
The copyright was injected into the public domain for other reasons, such as failure to adhere to required formalities or conditions;
The institution owns the copyright but is not interested in exercising control; or
The institution has legal rights sufficient to authorize others to use the work without restrictions.a