var img = document.getElementsByClassName("showcaseimg");
for (i = 0; i < img.length; i++) {
    img[i].onclick = function(){
        var interval;
        setInterval(function() {
            img[i].style.transform="scale(2,2)";

            window.onclick = function() {
                clearInterval(interval);
            }
        }, 3000);
    }
}