If you don’t take the time to enter in some ALT tags for your images, you might be missing the boat on a very powerful SEO tool. How powerful? Read Charles Arthur’s article on “Another Fiendish way to make Money online” and see if any lightbulbs turn on above your head. I’m not suggesting that you use the ALT tags in the same way as in Charles’ example, but it doesn’t take a genius to see the advantages that using full fledged ALT tags can give.
Imagine if you will, you’ve gone to the trouble of creating some graphs of the cost of blue widgets compared to the cost of red widgets over time with various marketing ploys. It becomes a very hot topic and many, many people link to the image or hot link it from your site. If you’ve used the ALT tags on the image, you’ve gained a number of links that have the exact anchor text you want on them.
If you’re not using ALT tags (and I have to admit that I sometimes don’t) you might be ignoring a perfectly good SEO/SEM tool!