Yes, in the United States there is a time-limit for the copyright of a photograph, even if the image has been copyrighted with the U.S. Library of Congress in Washington, D.C. How long a photographic image remains copyrighted depends on when it was made.
Copyright is possible by registering your photographic image with Library of Congress. However, this process is not necessary (U.S. copyright through the Library of Congress is still recommended by many experts), because the publication of the photograph itself establishes copyright.
Death Plus 50 Years
In 1976 the copyright law for photographs was revised by Congress. As a result any photographic material that was created or published after January 1,1976, now has a copyright that lasts until 50 years after the death of the photographer.
Before January 1,1976
Publishing a photograph before the 1976 law was implemented meant that the picture came under the old law, which stated that copyright was protected for 75 years from the time of publication, regardless if the photo artist is dead or alive.
In the 1976 ruling it was decided that unpublished photographs, such as those found in diaries and photo albums, have the same time limits as published photographs. Therefore any image made after January 1, 1976 has copyright protection for the life of the photographer, plus 50 years.
The Digital Age
The Digital Millennium Copyright Act, passed on October 1998, has very much changed the realities of copyright protection. In a nutshell, this Act of Congress extended all current copyrights (at the time the bill was signed), for another 20 years, thus putting a freeze on all photographic material that was published after January 1,1923. Any photograph that was published before 1923 would have entered the public domain (via the 75 year rule) before the 1998 law was passed, so the time extension does not apply to these photographs.
On the Web
Unless the rights have been given away by the artist, all images on the web are protected by copyright. Some popular viewing sites require the artist to give away rights in order to display, while others do not.