Unless they've been released into the public domain (which PR companies may do to promote films etc.), then yes the photos are copyrighted, and the sites that publish them I imagine are just taking the risk that the copyright owner isn't going to do anything about it.
I think it's safest to assume that unless you took the photo yourself or got it off a stock photo site, it's NOT in the public domain. I could be wrong (if I am, somebody tell me)
like have a disclaimer say something like "we at something something.com have no control over the uploaded content. by visiting this site you agree to ackowledge that something.com bares no responsibilty at all and will be liable for nothing and none whatsoever. if there is an image that is copyrighted, contact us and we will remove the photo. etc..."
no, you can still be sued for stealing content that doesn't belong to you... there is nothing in copyright law about disclaimers protecting you
Its so common to use pohotos of sports, fil people, I guess no one really minds. They do mind when the photo is much more valuable and rare. There are some photos that are so widely used that it becomes quite commoon to share. But the person using them should ask for permission nevertheless. They probbably don't ebven know who owns the rights to the, Itss probably one of the large phot agencies.
i think only if you decided to not take it down than you can sued. that is the purpose of disclaimers.
A summary of copyright and internet is here http://mason.gmu.edu/~montecin/copyright-internet.htm Although there are a lot of issues unresolved there is this website http://www.copyright.gov/circs/circ1.html#wci that resolves most of them. A simple unresolved question for example is: What happens in the case that a website has wallpapers that are created by different user and you just distribute them? You don't know the owner, you don't know the source. As you can see in my links there is a website of my friend full of wallpapers and almost 95% of them are sent by unknown users.
when I use product photos (or photos in general) on my websites I like to add a line under the photo that states where I got that photo and provide a link whenever possible. I know it doesn't really mean much but most people don't mind as long at they're given full credit and a link for their photo. Especially if it helps them sell something.