An excellent article on how and when to tag your images with alternate (ALT) text. Examples of good and bad practice help us understand how this image substitute text should be used. A must read for every web publisher.
It may surprise some to find that actually, sometimes an empty ALT text attribute is most appropriate. And sometimes, literally describing what is in the image isn't the best approach.
David Travis identifies five different types of ALT text, and gives examples and advice on what do in each case.
Five kinds of ‘alt’ text - article by David Travis