Zoom in and cut images in Twenty Seventeen header image

To Solve this issue better to use javascript library imageScale();

Different options available in this(fill, best-fill, best-fit, best-fit-down).

You have to just include js file. then after using the following format for the image.

<div class="image-container">
  <img class="scale" data-scale="best-fit-down" data-align="center" src="https://wordpress.stackexchange.com/questions/300845/img/example.jpg">
</div>

Now add this JavaScript code to your page.

$(function() {
   $("img.scale").imageScale();
});

The second option is to use another javascript imagefill.

<div class="container">
  <img src="https://wordpress.stackexchange.com/questions/300845/myawesomeimage" />
</div>


$('.container').imagefill(); 
  // image stretches to fill container

for more imageScale | imageFill