Background image is not responsive

Hello,

As you can see if you view the videos below. I am using the twenty sixteen theme. I have added a background image that needs to be full screen.
On a PC the background image is too large and doesnt fit the screen. As you can see below.

http://s31.postimg.org/z1itwozzv/wordpressbgpc.jpg

And on a mobile device the image background is too small and I would rather use overflow:hidden or something to make the image too big on mobile and fit the screen.

http://s31.postimg.org/lnulqekhn/image1_1.png

Can somebody please tell me how I can fix this?