For very well-known celebrities, the US government archives may have something: a Google image search on "site:.gov" may bring something up.  Also, the search engine on the Archives website (http://www.archives.gov/) may find things that Google doesn't.  For people with military service, the military might have something (Google search on "site:.mil").

For celebrities that were well-known before 1950, there may be newspaper photographs that have fallen out of copyright.  These will be hard to find, but the Library of Congress (http://www.loc.gov/) may have something.

A further option is to contact the celebrity's agent or estate and ask for an image to be released under GFDL, Creative Commons attribution, or Creative Commons sharealike-attribution licenses.

In my opinion, if free-use images can't be found, the second-best choice is a promotional photo: it's an image that is intended to be distributed widely.  Of course, there are exceptions: for albums, singles, and DVDs, a low-resolution scan of the cover is always appropriate.  For movies, a still from a well-known scene or a low-resolution scan of a promotional poster is good.  Failing all else, other fair-use images can be used, but they need to be tagged, justified, have the source/copyright owner credited, and used very sparingly.

As for the images at Iron Maiden, I'm still trying to figure out how strict I should be about fair-use images: all the images there are directly related to the subject of the article (except Image:PrisonerNovel.jpg, which I've just removed), all of them have some semblance of a source indicated, all of them have some semblance of a fair-use justification, and none of them seem to be surplus to the needs of the article.  I should probably push to have the justifications be made more explicit.