It depends on what you are doing with them.
For example: a "fan" site featuring a particular celebrity (or group, such as a band) is okay. Also, a "parody" is generally okay, like if you are spoofing Madonna by adding even more armpit hair. Check with
the U.S. Government regarding the exact nature of copyright protections for celebrities in general.
Note that if the photos are copyrighted by the photographers, themselves, or the news organizations that paid them to take the pictures (a "work for hire"), there are additional layers of copyright protection to wade through. But if you keep your site limited in its scope, as described above and in the text of Title 17, you should be okay.
As a blanket rule, you cannot simply post other people's photos and then charge for copies of those photos or charge others to view them, like by using some kind of paid registration system.