Hi,This page [1] lists a lot of different image categories and describes what type of alternative text to provide. The spec seems to cover most of the relevant categories already, though it should probably address advertising banners/interstitials and CAPTCHA images. If you do include CAPTCHA images, you should probably also reference the W3C note about them [2].
[1] http://html.cita.uiuc.edu/text/ [2] http://www.w3.org/TR/turingtest/ -- Lachlan Hunt http://lachy.id.au/
