Smashing Magazine recently posted an article that details how to make sure the alt text for an image does a sufficient job of ensuring it is accessible for everyone.
A valuable tool the article provides is the online image decision tree to help determine what kind of alt text is needed.
A few worthwhile snippets from the article...
But having alternate text is not enough — it must also be meaningful.
For more complex alternative text phrases, conduct the telephone test. For example, if you called up a friend and said “purple slug” and hung up the phone your friend would probably be confused, but also might think of a purple slug — but in what context? If you called a friend and said “the purple slug is eating my hydrangeas,” that would paint a more vivid picture — without adding a lot of additional characters or effort.
Of course, an assistive technology user will have to listen to your alternative text, so don’t go overboard. That is why it is suggested to cap your text at 150 characters. If you need to add more context to the image (e.g. complex image), there are other, more descriptive patterns or methods you can use to add more detail.