Preloading JS calculation prior to images loading

HTML

<!DOCTYPE html>
<html>

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Title</title>
    <link href="images/favicon.png" rel="shortcut icon">
    <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js">
    </script>
    <link href="styles.css" rel="stylesheet">
</head>

<script>
	
 window.addEventListener('load', function() {
            var headerHeight = document.getElementById('header').clientHeight;
            document.getElementById("content").style.paddingTop = headerHeight + "px";
            var footerHeight = document.getElementById('footer').clientHeight;
            document.getElementById("content").style.paddingBottom = footerHeight + "px";
        }, true);

        window.addEventListener('resize', function() {
            var headerHeight = document.getElementById('header').clientHeight;
            document.getElementById("content").style.paddingTop = headerHeight + "px";
            var footerHeight = document.getElementById('footer').clientHeight;
            document.getElementById("content").style.paddingBottom = footerHeight + "px";
        }, true);	
	
</script>	
	
<body>
    <h1>H1</h1>
    <div id="background"></div>
    <header id="header">
        <div id="logo"> <a href="/" class="nonblocklink"><img alt="A logo." src="images/logo.png"></a> </div>
        <div id="nav">
            <ul>
                <li id="projects">PROJECTS
                    <ul>
                        <li id="one"> <a href="one.html" class="blocklink" target="_self">ONE</a> </li>
                    </ul>
                </li>
            </ul>
        </div>
    </header>

    <div id="content">

        <div id="cards">

            <img src="images/back1.png" alt="" class="cards" />
            
            <img src="images/back2.png" alt="" class="cards" />
            
            <img src="images/back3.png" alt="" class="cards" />
            
           ...

CSS

::selection {
	background: #D5D5D5;
}

::-moz-selection {
	background: #D5D5D5;
}

h1 {
	font-family: Arial;
	margin-top: 10px;
	margin-left: 10px;
	padding: 0px;
	top: 0;
	left: 0;
	font-weight: bold;
	font-size: 15px;
	text-align: left;
	position: fixed;
	-webkit-touch-callout: none;
	-webkit-user-select: none;
	-khtml-user-select: none;
	-moz-user-select: none;
	-ms-user-select: none;
	user-select: none;
	pointer-events: none;
	color: #000000;
	z-index: -2;
}

#background {
	background-color: #FFFFFF;
	margin: 0px;
	padding: 0px;
	top: 0;
	left: 0;
	width: 100%;
	height: 100%;
	position: fixed;
	z-index: -1;
}

a.nonblocklink {
	text-decoration: none;
	color: inherit;
	display: inline-block;
}

a.blocklink {
	text-decoration: none;
	color: inherit;
	display: block;
}

#header {
	background-color: white;
	width: 100%;
	position: fixed;
	margin: 0px;
	padding: 0px;
	padding-top: 10px;
	top: 0;
	left: 0;
	text-align: center;
	z-index: 10;
}

#logo img {
	width: calc(100% - 20px);
}

#logo img:hover {
	-webkit-filter: invert(100%) !important;
}

#nav {
	width: 100%;
	background-color: white;
}

#projects {
	display: inline-block;
}

#nav ul {
	font-family: Arial;
	font-size: 15px;
	font-weight: bold;
	color: #000000;
	list-style-type: none;
	text-align: center;
	margin: auto;
	padding-top: 6px;
	padding-right: 0px;
	padding-bottom: 10px;
	padding-left: 0px;
}

#nav ul ul {
	width: calc(100% - 20px);
	list-style-type: none;
	font-weight: normal;
	display: none;
}

#nav ul li:hover>ul {
	display: block;
	position: absolute;
	text-align: center;
	margin: 0 auto;
	padding-top: 10px;
	left: 0;
	right: 0;
}

#one {
	padding-top: 10px;
	padding-bottom: 10px;
	border: 1px solid black;
	color: #000000;
	background-color: white;
}

#one:hover {
	background-color: black;
	color: white;
}

#footer {
	width: 100%;
	background-color: white;
	position: fixed;
	margin: 0px;
	bottom: 0;
	left: 0;
	text-align: center;
	z-index: 11;
}

#footer-nav {
	width: 100%;
}

#info...