Use of the longdesc attribute, to point to a page with a long description of an image:

Edit code and click me	Your Result (Some examples do not work in Google Chrome):
<html> <body> <img src="/images/w3html.png" alt="Xhtml.co.il" longdesc="/tryit/html/xhtml.txt" width="120" height="95" /> </body> </html>
Edit the code above and click to see the result	xhtml.co.il - Try it yourself