Photo gallery centering

Hello,

On page:

https://notredamedesgraces.be/testnewsite/?page_id=11

I am trying to use a wordpress gallery with photos as a menu. I have two issues: Gallery cannot be centered. And I cannot find how to use each photo to link to another page, only to the photo itself.

Can you please help? Do you know by any chance a plugin that does it easily? Nexgen wont work either, quite hard to center and to change image size…

Thanks!