Why are ALT tags important for Search Engine Optimisation?

ALT tags are used to provide Internet users with a description of an image on a web page. You'll see the ALT tag when you hover your mouse over the image. These tags are useful if an image hasn't loaded or if the browser has images turned off.

Here's an example of image that hasn't loaded:

This is the ALT text when the image doesn't load

As ALT tags increase the usability of a web site, they play a part in the assessment that a search engine will perform of the web page. The search engines that incorporate ALT tags into their search algorithms are Google, Alta Vista and Teoma.

ALT text is added to an image like this:

<img src="ALT-demonstration.gif" alt="The pop-up text is the ALT text">