Fantastic image alt attribute primer and tutorial. Use of the alt attribute, or alt text, is a simple step towards writing standards compliant and accessible HTML markup.

The first step when determining appropriate alternative text for an image is to decide if the image presents content and if the image has a function. In most cases, an image will only have a function if it is contained within a link. Determining if the image presents content and what that content is can be much more difficult. If the content that the image conveys is presented within text in the surrounding context of the image, then an empty alt attribute may suffice. In the example above, the content being presented by the image is to inform the user that this is George Washington. The image has no function because it is not a link and is not clickable.

Via 456 Berea Street.