For the main page, the subheader has a background image

<div class="ph-subheader ">
...
</div>

The image can be changed from app/css/scss/_site-header.scss

The subheader has the following code:

 <div class="ph-subheader" id="home">
  <div class="custom_width custom-height-container container flex">
   <div class="flex-center home-slide-content">
    <div class="title-block">
     <h1 class="home-heading">Organic natural food
     </h1>
     <p class="home-subheading">Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod</p>
    </div>
    <a href="index.html#menu" class="btn btn--black btn--30 btn-scroll">
      See our menu </a>
   </div>
  </div>
</div><!--end subheader-->