Help! trying to make a adjustable height div based on screen resolution

Help! trying to make a adjustable height div based on screen resolution

Yes, it is very compliqued, but i'm just doing this for one reason, I wanna learn but I was looking at various sources and found nothing to the solution of my problem. (sorry for my English, I am mexican)

I want to do something like this page:
http://www.avira.com/es/index

or

https://www.avast.com/index

I'm building my website and here is an example

What I want to do is that the image of the desert (or whatever it is) covers the entire screen. (height)

If you notice, in Avira and Avast, When you change the resolution screen, the image remains, I would do exactly the same.  but   the   problem   is   that   I   don't   know   how  to  do  it.

Please, I need an Answer