Why is my variable myImage coming up in the debugger as Null

Question

I am new to learning javascript and programming. I copied this snippet from a learning website. overtime I load the page I get an error in the debugger that myImage is null? Can someone help explain why??

Thanks!

var myImage = document.getElementById("mainImage");

var imageArray = ["_images/heretic.jpg","_images/prudential.jpg","_images/woofs_logo.jpg"
                ];
var imageIndex = 0;

function changeImage() {
    myImage.setAttribute( "src",imageArray[imageIndex]);
    imageIndex++;
    if (imageIndex >= imageArray.length) {
        imageIndex = 0;
    }
}

// setInterval is also in milliseconds
setInterval(changeImage,5000);