Yesterday Google announced in the Webmaster Central blog the added expansion for images within XML Sitemaps.
By expanding your XML Sitemap to include images in the new format, this will allow Google to easily learn which images you consider important. With image related queries being performed at Google every day, this extra step may help you to benefit from a bit of increased image related traffic.
The following is a snippet of code, with an example of an image inclusion:
More information on adding images to your XML sitemap can be found at Webmaster Tools Help.
Also be sure to check out “Optimizing for Google Images” for more information on how to get your images ranked well by Google.