JSFiddle - React, Tailwind, and code Playground

HTML

<img src="http://photo.is/o/10.gif" width="100" height="100" />
<img src="http://photo.is/o/20.gif" width="100" height="100" />
<img src="http://photo.is/o/30.gif" width="100" height="100" />
<img src="http://photo.is/o/40.gif" width="100" height="100" />

JavaScript

window.onload = function() {
        for (var i = 0; i < document.images.length; i++) {
            var image = document.images[i];
            image.onclick = function() {
                var images = Array.prototype.slice.call(document.images);
                alert(Array.indexOf(images,this));
            }
        };
    };