JSFiddle - React, Tailwind, and code Playground

HTML

<ul>
    <li><a href="index.html"><img src="http://s21.postimg.org/hzb9gge93/hp_homebutton.png" alt="Home" /></a></li>
    <li><a href="#"><img src="http://s21.postimg.org/bbentuuqv/hp_monthsbutton.png" alt="Months" onmouseover="this.src='http://s21.postimg.org/8ssyt690n/hp_months.png'" onmouseout="this.src='http://s21.postimg.org/bbentuuqv/hp_monthsbutton.png'" class="months" /></li>
    <li><a href="#"><img src="http://s21.postimg.org/rer4tnw9z/hp_forumbutton.png" alt="Forum" /></li>
    <li><a href="#"><img src="http://s21.postimg.org/ig12gmrdz/hp_newsbutton.png" alt="News" /></li>
</ul>

CSS

ul {
    list-style: none;
    margin: 0;
    padding: 0;
}

ul li {
    clear: both;
    height: 54px;
}

ul li img {
    position: relative;
    z-index: 1;
}

ul li .months {
    z-index: 2;
}