I am starting up a website that includes an online magazine and an RSS feed. Is it OK if I use pictures I pick up from other Internet sites for my reviews on products or services? What about general photos or pics of famous people?
I already have users in Europe and elsewhere. Is there any international law that applies to Internet sites? Are the copyrights registered in the respective country good only in that sovereign nation? Or does it cover globally?
Using the pictures you pick up from other websites without specific permission infringes the copyrights of whoever took the photos.
The sites may also be infringing on the photographer's or hirer's rights, but that doesn't make it OK. The rule for federal copyright infringement is always the same. Whoever took the photo, or hired the photographer or bought the copyright from the photographer or other owner, owns the copyright and can exclude all other users from use.
As a practical matter, would anyone sue over use of one photo? It's unlikely. But, if the photo has been registered for copyright and the infringement is willful, the liability can be $150,000 for each infringed work.
Regarding use of depictions of "famous people," all of the above applies. Also, the famous person has a state publicity right, in most states, to exclusively exploit their own image, signature, voice, etc. for commercial gain, and to exclude and sue over others' uses of their persona. If the photo is used for a legitimate news or sports story, then the law won't apply, but celebrity gossip generally isn't "news."
As to the international users, copyrights are territorial and a U.S. copyright is only good in the U.S. But, that said, most countries are all part of the same internationally recognized Berne Convention and WIPO treaties that reciprocate copyright protection to each other. So, someone who owns a U.S. copyright that's infringed in Argentina can probably sue in Argentina.