Ah, you mean the menu loads over the image and than is getting placed below the image? You could consider a simple preloader wich solves this problem.
If you do not want this, you can set a fixed height on the header part with css. Just make sure you load this inline (inside your template, not via an external css file).
<style>
#customizr-slider-1.parallax-wrapper { height: 300px; }
</style>
Place this code in the header.php of your theme, right after </head>. Don’t know if it will work but you can try 😉