JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8.2/themes/ui-lightness/jquery-ui.css">
<script src="http://ajax.microsoft.com/ajax/jquery.validate/1.7/jquery.validate.js"></script>
<div id='gallery'>

<div class='thumbnail'>thumb</div>
<div class='thumbnail'>thumb</div>
<div class='thumbnail'>thumb</div>
<div class='thumbnail'>thumb</div>
<div class='thumbnail'>thumb</div>
<div class='thumbnail'>thumb</div>
<div class='thumbnail'>thumb</div>
<div class='thumbnail'>thumb</div>
<div class='thumbnail'>thumb</div>
<div class='thumbnail'>thumb</div>
<div class='thumbnail'>thumb</div>
<div class='thumbnail'>thumb</div>
<div class='thumbnail'>thumb</div>
<div class='thumbnail'>thumb</div>
<div class='thumbnail'>thumb</div>

</div>

CSS

.last {
    background: yellow;
}

.bottom-last {
    background: orange;
}

JavaScript

$('#gallery .thumbnail').addClass(function(i) {
    return ((i == 4 || i == 9) && 'last') || (i > 9 && 'bottom-last');
});