Images sitemap not indexed

I have uploaded for a website two normal sitemaps and two images sitemaps (one for each language), respecting the structure given by Google.

While the normal sitemaps are indexed normally only very few images are indexed, and I can't figure out what's wrong.

Here is a simple record from the sitemap

<url>
    <loc>http://www.holypopstore.com/it/footwear-uomo/2304/nike-air-max-uptempo-95-as-qs</loc>
    <image:image>
        <image:loc>http://holypopdata.s3-website-eu-west-1.amazonaws.com/images/galleries/1646/922932-400.png</image:loc>
        <image:title>Air Max Uptempo 95 "All Star" university blue/white</image:title>
        <image:caption>922932-400</image:caption>
    </image:image>
</url>

Anyone can suggest me what is wrong (in the map or in page)?

Thanks in advance.