This is not new information but bears repeating.
Most SEO professionals know about creating XML sitemaps for web sites so that search engines can more easily find and index content. One limitation of sitemaps is the ability only to include URLs in the map – until now. Google has created an extension to XML sitemaps that is used to provide additional information about important images that exist on each page in the sitemap. This may be useful for logos or other branded images. See the sitemap code example below.
More complete information is available on the Google blog.