Do not assume that clearing the right to use a photograph is sufficient copyright clearance.  Even clearing a photograph of a subject in the public domain may not be sufficient, if a derivative work is the actual subject of the photograph you cleared. 

The U. S. Postal Service used a stock photograph of the Statue of Liberty to create a new stamp.  But the photograph was actually of the model Lady Liberty in Las Vegas --not the original in New York harbor.  Robert Davidson, the artist who created the model, upon seeing the stamp (after his wife came home from the post office and exclaimed "they put our statue on a stamp") registered the copyright in his version of the statue and sued.  After five years of litigation and a two week trial, the court awarded the artist $3,554,946.95 plus interest

A photograph of a work of art that is in the public domain may still be sufficiently creative, in choice of camera angle, lighting, cropping or design, to qualify for copyright protection.  Always think seriously about the photographer's rights.  But keep thinking.  Exactly what is the subject of the photograph?  Many subjects in the public domain are reproduced in many forms for many purposes.  Some may be a fair use by the second user, but your use may not qualify.  A derivative work is entitled to copyright protection for the copyrightable elements that the second user adds to the work.  The case is  Davidson v. the United States.