//Modify this to the total amount of images you want to randomly cycle through.
var numImages = 12;

//No need to change this line.
var number = (Math.round( (Math.random() * (numImages - 1) ) + 1) )

//You may need to change this line depending on what your images are called.
//This assumes that they are image1.png, etc.
var htmlsource = "<img src = intro_" + number + ".jpg>";

//No need to change this.
document.write(htmlsource);