What exactly is the intended use of the alt attribute on <img> tags?
Should it describe the image, or should it provide meaningful replacement text for screen-readers (and people who have images turned off)?
For example, if I have a short biography of a person on my website, and include a small photo of them, is it really meaningful to visually impaired users to have "Photo of John Smith" read out to them?
The HTML 5 spec has a huge section on this.
Basically, the alt attribute should be what would be there as text if you hadn't used an image at all. If your image is purely decorative, you can use alt="". A description goes in the title attribute.