index.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Random Image y Reloj</title>
</head>
<body>

<img id="imagen" style="background-size: cover">

<script>
    arr = ["https://i.picsum.photos/id/1024/1920/1280.jpg", 
    "https://i.picsum.photos/id/1019/1920/1280.jpg", 
    "https://i.picsum.photos/id/1020/1920/1280.jpg",
    "https://i.picsum.photos/id/10/1920/1280.jpg",
    "https://i.picsum.photos/id/1043/1920/1280.jpg",
    "https://i.picsum.photos/id/1051/1920/1280.jpg" 
    ];
let imagen = document.getElementById('imagen')
function imagenRandom (){
  imagen.src = arr[Math.floor(Math.random() * (arr.length))]
} 

window.setInterval(function(){
  imagenRandom(arr)  
}, 5000);

</script>
</body>
</html>