body{
  margin:0;
  padding:0;
}

section{
  position:absolute;
  top:0;
  left:0;
  width:100%;
  height:100%;
  background:url(https://image.freepik.com/free-photo/sport-composition-with-modern-elements_23-2147914163.jpg);
  background-attachment:fixed;
  background-repeat:no-repeat;
  background-size: 100% 100%;
}

section:before{
  content:'';
  position:absolute;
  top:0;
  left:0;
  width:300px;
  height:100%;
  background:url(https://image.freepik.com/free-photo/sport-composition-with-modern-elements_23-2147914163.jpg);
  background-attachment:fixed;
  background-repeat:no-repeat;
  background-size: 100% 100%;
  box-shadow:20px 0 100px rgba(0,0,0,0.2),
              -20px 0 100px rgba(0,0,0,0.2);
  background-size:110%;
  animation: animate 5s ease-in-out infinite;
}

@keyframes animate{
  0%,100%{
    left:0px;
  }
  
  50%{
    left:calc(100% - 300px);
  }
}