I am trying to add the <image> tag to my sitemap, but even trying to look at the XSD , as mentioned by Google at  http://www.google.com/support/webmasters/bin/answer.py?answer=183668 , I get 404 Error not found (the XSD link: http://www.sitemaps.org/schemas/sitemap-image/1.1 ). How is that possible? Is there anyway I can validate it without the XSD?
Validating sitemaps with images tag
sitemap;validation