On the site:
  http://www.ngv.vic.gov.au
I am trying to find all the Web pages that have the term 
  viscopy
in the alt text.

How do I search for terms in alt text?

The NGV puts art work captions into the alt text of images.  I thought
that I would be able to use advanced search on Yahoo, Google or Teoma
to find terms in the alt text.

However, when I tested them with a unique term, such as "regattas"
(which appears in the alt text on
http://www.ngv.vic.gov.au/impressionists/06.html ), I get no
responses.

Any advice greatly appreciated.