Your images should be described using ALT Tags with the keywords you are optimizing for.
Instead of just putting
you should put it as
<img src=”image.jpg” alt=”Image descriptions for engines to index”>
Besides for SEO optimization, the purpose of ALT text is also to describe the image for the benefit of those who surf the Web with images turned off. Therefore, do not include keywords that are not related to your image. Keyword spamming will adversely affect your rankings in the SERPs.