How can I see all photos of one country?

You just need to go to the country page. You can search for it on the search box or if you're in a page where you see materials from that place, you can click on the country name directly.

Once there, click the icon with the number of photos that appears in the first box.

Additionally, a link pointing to the same page is placed at the bottom-right corner of the photos box. It says "View, sort and filter all photographs".