Sitemaps has truly been a basic SEO step for many large or even medium sized web sites. Google Image search has been a strong driver of traffic to many sites, but sometimes it is hard for webmasters to get their images indexed. Google announced support for images in Sitemaps.
Here is an example:
<?xml version="1.0" encoding="UTF-8"?>
With this you can specify image, location, caption, geolocation, title and license. For more details, see this document.
Forum discussion at WebmasterWorld.