Interesting, read this on the Google blog
[QUOTE]Adding Images to your Sitemaps
Posted: 2010-04-07 13:32:07 UTC-07:00
Webmaster Level: All
Now you can use a Sitemaps extension to provide Google with exactly this information. For each URL you list in your Sitemap, you can add additional information about important images that exist on that page. You don’t need to create a new Sitemap, you can just add information on images to the Sitemap you already use.
Adding images to your Sitemaps is easy. Simply follow the instructions in the Webmaster Tools Help Center or refer to the example below:
We index billions of images and see hundreds of millions of image-related queries each day. To take advantage of that traffic most effectively, take a moment to update your Sitemap file with information on the images from your site. Let us know in the Sitemaps forum if you have any questions.
Posted by Alkis Evlogimenos, Software Engineer[/QUOTE]