Alt tags ------------------------------------------

The ALT attribute is designed to be an alternative text description for images. ALT text displays before the image is loaded (if it's loaded at all) in the major browsers and instead of the image in text-based browsers like Lynx.
 
ALT is a required element for images and can only be used for image tags because its specific purpose is to describe images. Alt tags are also used in major search engine algorithms. Search engine robots or spiders cannot read images;
however they can and do read alt tags. Alt tags help search engines learn more about your website, especially those that lack a lot of content. If used correctly it will even help your website rank higher. Good and descriptive alternative text should be placed on all images, pictures and buttons according to the Design and Content Google Guidelines.

Benefits of good ALT and TITLE text:

-Enhance the browsing experience of visitors with disabilities.
-Increase your page's keyword density score and relevancy for your targeted keywords. (Calculate your page's keyword density with Page Primer and find out if you've used your keywords too little - or too often.)
 - Provide valuable information to all your visitors with descriptive link titles and descriptive text about other page elements.
-Required for site validation
-Accessibility issues its a plus! -Doesn't add much more content development time.

A good overview of application: http://www.feedthebot.com/titleandalttags.html

A good, slightly technical read: http://www.evotech.net/blog/2007/08/alternitive-text-for-images-the-alt-... A bit long and dry, but important: http://www.seoworkers.com/seo-articles-tutorials/alt-attribute.html