<div class="content">
  <p class="photo-p">
    ウインドウ幅の50%(calc + %)<br>
    <img src="https://placehold.jp/500x150.png" alt="" /></p>
  
  <p class="photo-vw">
    ウインドウ幅の50%(vw)<br>
    <img src="https://placehold.jp/500x150.png" alt="" /></p>
</div>
.content {
  padding: 0 10px;
  p {
    margin: 0;
    padding: 10px 0;
    text-align: center;
    line-height: 1.5;
    color: #fff;
    font-weight: bold;
  }
  .photo-p {
    background: #1253A4;
    img {
      width: calc((100% + 20px) / 2);
    }
  }
  .photo-vw {
    background: #F26964;
    img {
      width: 50vw;
    }
  }
}
View Compiled
$('ul').slick({
  arrows: false,
  autoplay: true,
  dots: false,
  infinite: true,
  speed: 300,
  slidesToShow: 1,
  centerMode: true
});

External CSS

  1. https://cdnjs.cloudflare.com/ajax/libs/slick-carousel/1.6.0/slick.css

External JavaScript

  1. https://cdnjs.cloudflare.com/ajax/libs/jquery/3.2.1/jquery.min.js
  2. https://cdnjs.cloudflare.com/ajax/libs/slick-carousel/1.6.0/slick.min.js