Due funzioni che si potrebbero adattare alle tue necessità.
codice:
<div id="mydiv" style="height: 450px;">my div</div>
<script>
const myDiv = document.getElementById('mydiv')
function rndBg1() {
const rnd = Math.random() * 3 + 1 | 0
myDiv.style.backgroundImage = `url('./images/bg-${rnd}.jpg')`
}
function rndBg2() {
const images = [
'./images/bg-1.jpg',
'./images/bg-2.jpg',
'./images/bg-3.jpg'
].sort(() => Math.random() - .5)
myDiv.style.backgroundImage = `url(${images[0]})`
}
rndBg1()
// rndBg2()
</script>