The alt should work by default for your product images.

If you have other images in the raw HTML (such as for your product descriptions), you'll have to add the alt and title text like so:

<a href="http://www.lulu.com/content/466605"><img src="images/large/e-start-book.gif" alt="get your manual today" title="Have you got yours yet? Join the 1000’s of Zen Cart users that have bought the only comprehensive owners manual !" /></a>