Responsive problem

I use streched pages for gallery, and I cannot get is correct on mobile phone.

I am adapting tewentytwelve theme and have good result on computer :
https://www.lgphotography.fr/Help/FaunaOk-twenty.jpg

But the thumbnail size does not adapt to the screen for mobile phone:
https://www.lgphotography.fr/Help/IMG_20151125_183219.jpg

I acctually have tried with a lot of themes and find only one (I was using before) on which it really is responsive (snapshot plus).
Here is the result on the computer: https://www.lgphotography.fr/Help/FaunaOk.jpg
And on the mobile phone: https://www.lgphotography.fr/Help/IMG_20151125_183628.jpg

Here is the gallery link: https://www.lgphotography.fr/wordpress/portfolio/fauna/

Any tip?

https://www.ads-software.com/plugins/foogallery/