Responsive functions on the site

What is the easiest way to add a small responsive function on the website where for instance the pictures of the speakers would be 3 pictures next to each other on a big screen and when you watch it on a mobile device you see only one picture on the same line.

Now I added the pictures in different cells, which does not make it responsive.
http://www.kompetensinstitutet.se/smartaintranat/