Thumbnails and pictures don't display

Hello,

I use mantra theme (1.9.9.6).
I looked for 2 hours on the forum and the FAQs where I didn’t find anything. The problem occurs when I download the pictures either from my computer or from the media library! (With the theme’s gallery it works normaly although the design is very bad)

I would be glad if you have any idea!

You can see what is does on this page
https://know-mansland.com/2013/03/10/essai-galerie-lalbanie/

Thanks
Boris.

https://www.ads-software.com/extend/plugins/nextgen-gallery/