jQuery addClass example
Change class name on click in jQuery
HTML
<div id="banner-message">
<a class="IndexImage" href="/forum/discussion/55/design-flaw-001-give-up"><img src="https://www.w3schools.com/w3css/img_lights.jpg" class="IndexImage" id="yourImg"></a>
<button>Change image source</button>
</div>
JavaScript
var button = $("button")
button.on("click", function(){
$('#yourImg').attr('src','https://www.w3schools.com/w3css/img_mountains.jpg');
})