var1="pic1.gif"
var2="pic2.gif"
var3="pic3.gif"
date=new Date()
num=(date.getSeconds())%3
num=num+1
quot="'"
document.write("Случайная цифра:"+ num + "<br>")
document.write("<img src=" + quot + "pic" + eval(num) + ".gif"+ quot + ">")

<script>
let var1="https://via.placeholder.com/150/FF0000/FFFFFF?text=1"
let var2="https://via.placeholder.com/150/006400/FFFFFF?text=2"
let var3="https://via.placeholder.com/150/0000FF/FFFFFF?text=3"
let date=new Date()
let num=(date.getSeconds())%3;
let img = [var1, var2, var3][num];
num=num+1;
document.write(`Случайная цифра: ${num}<br>`)
document.write(`<img src="${img}" alt="">`)
</script>