Find in your html file the preloader-container block. Replace the svg tag with an image tag and set the path to your image.
Before:
<div class="preloader-block"> <div class="preloader-container"> <svg id="Layer_1" x="0px" y="0px" width="600px" height="690px" viewBox="0 0 600 690" enable-background="new 0 0 600 690" xml:space="preserve"> <polygon fill="#FFFFFF" class="polygon" stroke="#A7A9AC" stroke-width="8" stroke-miterlimit="10" points="54.558,557.559 300,132.441 545.441,557.559 "/> <polygon fill="#8856A3" class="inner" points="96.037,535.115 300,168.885 503.963,535.115 "/> </svg> </div> </div>
After:
<div class="preloader-block"> <div class="preloader-container"> <img src="preloader.jpg" alt=""> </div> </div>