Providing text equivalents for non-text elements on the web is one of the top priorities identified in the W3C Guidelines for making web pages accessible to all users.

The ALT attribute within the <IMG> tag allows you to do this for images. As well as making your site more accessible, it also makes your site ’search engine friendly’ because the search engine spiders can identify your images.

The LONGDESC attribute can also be used to provide more detail about a non-text element. The W3C states that the LONGDESC attribute can be used with the <IMG> and <FRAME> tags. (The LONGDESC attribute provides a link to a page providing further information - I’m not sure how well this is supported by different browsers). The ALT attribute can still be included to provide a title for the image.