Unlike humans, search engines don’t see images, so the more image rich your site is the less likely a search engine will understand its content. This is a predicament for any web developer/owner because they want to have a website that stands out from its competition and is aesthetically pleasing, showing off the products/services they are selling to the best of their ability. A website without images would be to most people a rather boring site to be on but how do you achieve a site that is not only pleasing on the eye but also manages to rank well in the search engines? Fear not, there is one SEO element that can easily be implemented as well as getting the opportunity to reinforce your keyword phrases. Assigning ALT text to all the important images on your site will enable the search engine to ‘see’ the image and understand its content thus ranking it accordingly.
Add the ALTernate text to the HTML tag so this is displayed by the browser when a user scrolls the cursor over the image. This is also good practice for any web owner to aid the visually impaired who views your site. Contributing to usability and accessibility always gets a thumbs up and by implementing this strategy you are not only providing more content for the ‘spiders’ but also the users as well.
Tip: Keep ALT text descriptions as informative and closely related to the image subject as possible and keep it short and sweet.
You can read more at http://www.seomoz.org/ugc/dont-forget-to-optimize-for-images-including-pdfs