Many SEO novices limit their optimisation efforts to the words on their web site, but the images on your pages can be, to coin a phrase, worth a thousand words.
At the most basic level adding Alt Text for every image on your site helps. But there are host of extra considerations you may wish to bear in mind, as Danny Dover of SEOmoz.org explains in this video: http://www.seomoz.org/blog/image-seo-basics-whiteboard-friday