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body::after{
    position:absolute; width:0; height:0; overflow:hidden; z-index:-1;
    content:url(img01.png) url(img02.png) url(img03.png) url(img04.png);
}

/* preloader */
div#preload {
    display: none;
}
<!-- preloader -->
<div id="preload">
    <img src="img/bg/BG1.jpg" width="1" height="1" alt="Image 01" />
</div>