normcdn on "Google only index thumb images ?"

Hi,
I have been using Coppermine for many years. Recently I discovered ZenPhoto that it is more appropriate for my recent photo album (and eventually all my photo albums). I installed the script and integrated into my website. Everything is fine with ZenPhoto.
There is something that bugs me. I use XML-Sitemaps v6.1 to create sitemaps. It works great as it indexes all my webpages and images … and even ZenPhoto images. I submitted sitemap.xml and sitemap_images.xml to Google Webmaster Tools and few days later I can see results by typing in Google image … site:cayoparedon.net
The thing I am not happy with, is Google ONLY displays thumbnails of my ZenPhoto album and not the actual images ???? In my sitemap generator configuration, I disabled files containing « _thumb » and the result is exactly what I need as you can see in … http://www.cayoparedon.net/sitemap_images.xml It shows the caption and filename. Perfect! BUT WHY Google picked the thumb images instead of the normal size images (500x375 pixels) in the sitemap file that I submitted ?????
Thanks for your help!