Understood:
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/meyer-reset/2.0/reset.min.css">
<style>
#custom{
width: 100%;
height: 100%;
//border: 1px solid black;
background-image: url('');
//background-position: center center;
//background-size: cover;
//background-repeat: no-repeat;}
</style>
<body>
<body onload="changecustombg()"><div id="custom" ></div>
<script>
function changecustombg(){
var random= Math.floor(Math.random() * 6) + 0;
var customImages = ["url('https://media1.giphy.com/media/l0HlVrqPItQH8D4iI/giphy.gif')",
"url('https://ry3yr.github.io/OSTR/Diarykeepers_Homepage/extrafiles/images/bgfiles/cool_stuff_anim.gif')",
"url('https://ry3yr.github.io/OSTR/Diarykeepers_Homepage/extrafiles/images/bgfiles/cool_stuff_anim.gif')",
"url('https://ry3yr.github.io/OSTR/Diarykeepers_Homepage/extrafiles/images/bgfiles/comments_anim.gif')",
"url('https://ry3yr.github.io/OSTR/Diarykeepers_Homepage/extrafiles/images/bgfiles/comments_anim.gif')",
"url('https://media1.giphy.com/media/l0HlVrqPItQH8D4iI/giphy.gif')"];
document.getElementById("custom").style.backgroundImage=customImages[random];}</script>
Option #2
<style>
.custom_bg{
height: 100%;
width: 100%;
position: relative;}
</style>
<section><div class="rt-container">
<div class="col-rt-12">
<div class="Scriptcontent">
<div class="custom_bg"></div>
</section>
<script src='http://ajax.googleapis.com/ajax/libs/jquery/1.8/jquery.min.js'></script>
<script>function displayBG() {var bgm = ['https://i.ibb.co/9mxH8pL/Castillo.gif',
'https://i.ibb.co/fkK7bK2/Elec-Tower.gif',
'https://i.ibb.co/SxXkFBg/Mayl-EXE2.gif',
'https://i.ibb.co/3C8pd4h/Standart-Comp-EXE3.gif',
'https://i.ibb.co/LJKnfhh/Scilab.gif'];
$('.custom_bg').css({
'background' : 'url('+ bgm[Math.floor(Math.random()*3)] + ') repeat',
'background-position' : '100%',
'background-size' : 'repeat'});}displayBG();</script>
Option #3
<style type="text/css">
#header {
height:100%;
background:transparent url(https://i.ibb.co/SxXkFBg/Mayl-EXE2.gif) repeat cover cover;
}
</style>
<script type="text/javascript">
if (document.getElementById) { window.onload = changeHeaderImg };
function changeHeaderImg() {
var numimages=7;
rndimg = new Array("https://i.ibb.co/SxXkFBg/Mayl-EXE2.gif", "https://i.ibb.co/SxXkFBg/Mayl-EXE2.gif", "https://i.ibb.co/fkK7bK2/Elec-Tower.gif", "https://i.ibb.co/SxXkFBg/Mayl-EXE2.gif", "https://i.ibb.co/fkK7bK2/Elec-Tower.gif", "https://i.ibb.co/SxXkFBg/Mayl-EXE2.gif", "https://i.ibb.co/SxXkFBg/Mayl-EXE2.gif");
x=(Math.floor(Math.random()*numimages));
randomimage=(rndimg[x]);
document.getElementById("header").style.backgroundImage = "url("+ randomimage +")";
}
</script>
</head>
<body >
<div id="header"></div>
</body>