I know that generally images that are used on sites legally are stock photos purchased online. If you have a site; however, that features a famous person, where are you supposed to get photos from legally?
Well, you can get photos from professional freelance photographers that sell their pictures... news agencies that do the same (like the AP or UPS)... and from anyone else who has taken a picture and is willing to sell it. Oh, and you can take the photo yourself. Some fansites also send mail to the celebrity, asking for a headshot/signature. Then they scan it once received. But, like all other images used for other purposes, these images are protected via copyright and require the same permissions as images from other sources.
I think in most jurisdictions, the photographer holds the copyright and not the subject of the photo. As suggested by jigordon, you can take the photo yourself. Perhaps during press conference, movie launching or any situation wherein people would not think of you as stalker.
There is the GNU GPL which is used for various free/open source software projects. Creative Commons is the only one I know of for images but there are tons of either CC photos available outside of flickr.
Be careful! "Legal" is not a single-faceted issue. When many people ask about "legally" using a photo, they are thinking about copyrights, but at least in the U.S. there are also privacy and publicity rights. When a photographer who licenses a photo (whether a Creative Commons-type license or some paid license), he is only licensing whatever rights he himself has. Unless he has a model release that allows commercial use of the subject's image and is assignable, you probably have no right to use the image commercially, even if you have a license of the copyright. The same goes for photos you take yourself. Just because you own the copyright to a photo you take of a celebrity, it does not follow that you can use the photo for commercial purposes.
Yes, privacy issues are a concern. But the OP was talking about celebrities, who no longer have the general right to privacy. So generic privacy and publicity rights no longer apply.
I don't know what you mean by "generic privacy and publicity rights" but believe it or not celebrities do have rights, too. True, any public figure has a higher hurdle than the ordinary man in the street and, in the case of privacy rights, may have surrendered substantially all hope of claiming any rights to privacy. On the other hand, famous people may have an even stronger argument against someone who attempts to commercially exploit their image. No one has ever asked me for a commercial endorsement, and if someone used my image in connection with the sale of a product, I would be hard pressed to show that I suffered any economic damage. Tiger Woods, on the other hand, makes millions off his image and endorsements. Anyone who attempts to use Tiger's image in a commercial manner for free undermines Tiger's right to profit from his own persona. The OP didn't specify how he intends to use the photos. If his purpose is to illustrate articles about the celebrities on a news/editorial website, then the celebrities are probably fair game. If his purpose is to sell posters of the celebrities on his website, he had better have a very good lawyer and deep pockets.
I was asking in response to just simply posting a couple pictures mixed with articles on a site. The only capitalization would be via adsense ads.