How to make white stripes on the sides of slides in Slick Slider?

Question

JavaScript

Hello, I have a Slick slider with images of different sizes. How can I make the slide stay the same height, just white stripes appear on the sides. Here is the slide code:

$('.popup__slider').slick({ // Подключение Slick Slider
      adaptiveHeight: true,
      slidesToShow: 1,
      slidesToScroll: 1,
      arrows: false,
      fade: true,
      asNavFor: '.popup__slider_nav',
      responsive: [{
      breakpoint: 560,
      settings: {
         arrows: true
      }
 }]

Now the slider looks like this:



Thank you.