The ALT attribute of the <IMG> tag provides a text alternative for the images (and other non-text elements) used in a web page. Search engine spiders can’t interpret images (yet) but they can read the text you supply in the ALT attribute.

This is good from an SEO point of view and for accessibility. ALT text is read by the screen readers used by blind and partially sighted people. For this reason, if a site uses images in many different ways, i.e. logos, photos and buttons, it’s a good idea to stick to a naming convention throughout. For example, begin the ALT text for all photos with “Photo:” and buttons with “Button:” etc.

It can be tempting to stuff the ALT attribute with keywords, since it is read by search engines but not seen on-screen. However, this is easy for the search engines to detect and penalise, and it certainly doesn’t help people using screen readers. The ALT attribute should be used intended and provide an alternative text description readable by both people and spiders. Accessible coding and providing more text for the search engines to chew over will naturally improve a site’s ranking.

What is good for the SEO of a site is if different ALT text is used for the same image on different pages. For example, the logo on the homepage of a carpentry site could be “Logo: Smith and Jones - Professional Carpentry”. On other pages it might be, “Logo: Smith and Jones - Hand Made Wardrobes” or “Logo: Smith and Jones - Bespoke Bedsteads”.

For complex images, the LONGDESC attribute can be used to provide a more detailed description. The ALT attribute can still be included to provide a title for the image.

The ALT and LONGDESC attributes can also be included with other HTML elements such as the <INPUT>, <FRAME> and <APPLET> tags.

See the accessibility priorities identified by the W3C for more details.