You have a layout set at 1600px wide, which is not very practical. I recommend you read up on "responsive design", which involves making sites more flexible and adaptable to different screen sizes. If you use, say, a % width for theose divs (e.g. width: 80%; margin: 0 auto;) you can then tell the background image always to fill the div with background-size: cover;
But as I said, you have a bi of reading to do.