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This is the code my designer gave me, but I don't know where to put it so it works:

<html>

<head>

<style>

.customHeader{margin:0 auto;}

.img-responsive{

  display: block;

  max-width: 100%;

  height: auto;

}

@media (max-width: 767px) {

  .hidden-xs {

    display: none !important;

  }

}

@media (min-width: 768px) and (max-width: 991px) {

  .hidden-sm {

    display: none !important;

  }

}

@media (min-width: 992px) and (max-width: 1199px) {

  .hidden-md {

    display: none !important;

  }

}

@media (min-width: 1200px) {

  .hidden-lg {

    display: none !important;

  }

}

</style>

</head>

<body>

<div class="customHeader">

<img src="https://www.vanessakier.com/wp-content/uploads/2017/12/VanessaKier-banner-1500x500.jpg" alt="Vanessa Kier" width="2000" height="500" class="img-responsive hidden-xs hidden-sm"/>

<img src="https://www.vanessakier.com/wp-content/uploads/2017/12/VanessaKier-banner-tablet.jpg" alt="Vanessa Kier" width="1000" height="350" class="img-responsive hidden-xs hidden-md hidden-lg" />

<img src="https://www.vanessakier.com/wp-content/uploads/2017/12/VanessaKier-banner-phone.jpg" alt="Vanessa Kier" width="767" height="300" class="img-responsive hidden-sm hidden-md hidden-lg" />

</div>

</body>

</html>