https://support.google.com/webmasters/answer/178636?hl=ru

<?xml version="1.0" encoding="UTF-8"?>
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"
        xmlns:image="http://www.google.com/schemas/sitemap-image/1.1">
  <url>
    <loc>http://example.com/primer.html</loc>
    <image:image>
      <image:loc>http://example.com/kartinka.jpg</image:loc>
      <image:caption>test</image:caption>
    </image:image>
    <image:image>
      <image:loc>http://example.com/photo.jpg</image:loc>
    </image:image>
  </url>
</urlset>