Image searches on the Web used to be awful. In 1998, if you searched for a photo of Phil Donahue, for example, you would get photos of every Phil and every Donahue, but not necessarily Phil Donahue, the talk show legend. Now punch him in to images.google.com, and you will see he's easier to find (though the technology is still not perfect).
That brings us to the question of ethics. There are plenty of places to search for and find photos, but that doesn't mean you should just put them on your publication or Web site. The lists below are suggestions for sites that allow you to acquire images for free or a fee. Be sure to read each site's rules.