add class with JavaScript

I am writing some vanilla JavaScript to create a nice navigation menu. I am stuck on adding an active class.

I am getting elements by class name NOT by id. The below code works if substituted with id, however, I need it to apply to more than one element.

HTML

<img class="navButton" id="topArrow" src="images/arrows/top.png" />
<img class="navButton" id="rightArrow" src="images/arrows/right.png" />

JS

var button = document.getElementsByClassName("navButton");

button.onmouseover = function() {
    button.setAttribute("class", "active");
    button.setAttribute("src", "images/arrows/top_o.png");
}

No answers containing jQuery please.