To create a custom loading for the slider, you just need to add its markup as a direct child element of the slider element with ms-loading-container class name. Below you can find an example:
  	<div id="masterslider" class="master-slider">
      <div class="ms-loading-container">
        <img src="../path/to/loading.gif" alt="">
        <!-- or any other form of loading -->
      </div>
      <div class="ms-slide">
        <!-- slide content here -->
      </div>
      <div class="ms-slide">
        <!-- slide content here -->
      </div>
      <div class="ms-slide">
        <!-- slide content here -->
      </div>
    </div>