Google, bing, yahoo all have image search engines. With careful consideration of images, for example info graphics, it is possible for sites to get visitors and clients from these searches.
Optimizing for image search has only a few variables; image name and URL, alt tag, page content near the image.
To SEO an image consider the keywords in these variables. Then consider how many different pages you can place (link to the image) the image on, places where it will be seen by many visitors. Can it be used on forums? Google places image? If the answer is yes then you can quickly give the image an importance for the keywords you desire. Most images on the internet are not well linked to.