It's not properly configured.
First of all images are managed by javascript on your website, only 1 real <img> tag is included in the page, this requires the special feature of recognize galleries images or custom images.
Moreover if you are not using precaching only a subset of products links are crawled based on the parameter 'Images/Videos sitemap requests limit' to avoid server timeouts.
You have activate the recognize galleries images and start a precaching process for images sitemap to crawl all links, there is a video tutorial at this link too:
https://www.youtube.com/watch?v=lTvnoiFXPKk&index=1&list=PL6t5xIMeFJ2JG3OQASXPLi5YrFduVGWq3
Best regards
John