Why is the background image cut off at the edges?

Question

css

Greetings! There is a demo site in the header of which a background parallax image is placed, but the problem is that it is cut off at the edges due to an incorrect height-width ratio (even background-size:cover does not help.
I'm trying to fix it using a script by setting the height-to-width ratio , but I get the effect is not the one I expect - what to do?

( function( $ ) {
  $('header').height($('header').width()/1.5);
  $(window).resize(function(){
    $('header').height($('header').width()/1.5);
  });
} )( jQuery );