How to display fullwidth images on mobile devices?

My theme Goodnews has been great so far, except for the fact that the images are not fullwidth, I don't like seeing a border white space as I scroll down on my mobile. How can I make all of my images responsive using my css child theme? I've tried some codes, but none of them has worked. My website is: http://ift.tt/1IfNXCW

I appreciate your help

Greetings

Tatiana