Resizing Mobile Gallery

Site URL: https://www.cinekhoi.com

Hello, 

I think I have the most basic question across all the forum that many have had before, but I have yet to find a solution. I've tried many codes I've seen floating around online, but nothing is working. 

What I need done is the ability to resize gallery images on mobile version only. 

I've attached the photos below, one of the desktop and then the mobile view. I would like to make the mobile gallery size smaller so it can show the whole image like it does on desktop. 

Thank you for all the help (if there's an answer already on another post, please link me). 

@tuanphan I saw you helping out on many posts, hoping you might know the answer to this?