Accessibility Is Important

If you or your nonprofit is concerned about your pages being readable to people with limited sight or non-graphical browsers, you should check out this classic book on accessibility. Building Accessible Websites explains in clear, detailed steps how to get your site up to speed. And it doesn't have to mean a major foray into tangled html. Just add some alt tags and title tags, and you've cleared the first hurdle. From Chapter 6:

The importance of making absolutely all the images on your Website accessible cannot be underestimated. This simple action alone gets you more than halfway toward an accessible site. You have to do it correctly, but even if you skip every other accessibility step, your sites immediately become fundamentally accessible.

Unfortunately, hundreds of major sites have not caught on to this and persist in using images for navigation without providing alt text or title text. It's obviously not good business to turn away disabled customers, and in some cases (as in web sites made by the government) it may be illegal. And besides, it is always good for search engines to have your images described in the text.