Use the long description (LONGDESC) attribute of an image to point to a Web page that contains a detailed description of the chart or graph.
We recommend that you use the Description Link (D-Link) since some browsers don't support the long description attribute.
Example LONGDESC code:
<IMG SRC="piechart.gif" title="48% female and 52% male Web users." longdesc="piechartdescription.html">
In this situation, certain browsers (and potentially, future ones) would give the user the option to see the description page so that they can read what the image is about.