Images on your web pages should all have “alt tags” in the code, which describe what the images are. This is for those who can’t see them, either through physical impairment or because their browser doesn’t show them. However, Google does index the content of these tags, so there’s also an SEO reason for ensuring your images have alt tags. If you use a content management system of any sort, it should allow you to specify the alt tag for any image as you insert it into a page. If your site is just a collection of pages created in something like Dreamweaver, you’ll need to add the appropriate code to each image.
The excellent Hobo blog has recently been looking at the maximum length of an alt tag. In How Many Words In ALT Text For Google, Yahoo & Bing? the author concludes that 16 words of 7 + 8 characters each seem to be indexed by Google, which I reckon means it stops indexing at around 130-140 characters. Plenty of room then to get your important search terms into the tag as well as describing the image. Remember though: don’t spam the alt tag with nonsense. It’ll come back to bite you.