We all want our websites to look cooler with images, video and... oh that ever so fun and useful Flash. Unfortunately all of these mean nothing to search engines, at least presently they don't. Luckily, one of the best and easiest things to do is to use the alt attributes or add alt tags to your images.
Basically, an alt attribute provides a place to describe what is in the image/picture and what it is about. The alt tags are a perfect place to put related keywords.
How does the alt attribute work for SEO?
When the search engine spiders are scanning your site, they will read the alt text and take it to mean what that picture's content is. The spider will then include it in the rankings and keyword indexing. But this is where black hat techniques get used and where your site could get penalized if you use them.
Avoid stuffing keywords into your alt tags just to rank for certain terms. Of course use the keywords which are going to help your site, but make sure they match the image. You can take a chance on stuffing but if a human screener ever reviews your site, you will be penalized.
Alt tags are also essential for screen readers, a software that reads a page aloud for visually impaired users, to describe what is on the page. These screen readers are another reason why you should avoid filling your image's alt text with keywords which don't have any bearing on what the picture is about. It would probably be quite annoying if a page is being read to someone and then all of a sudden a bunch of keywords are being blurted out instead of a proper description of an image.