<style>
    a.button {
        display:block;
        height:30px;
        width:184px;
        background:url(http://i.imgur.com/VBam6AY.png) no-repeat 0 0;
    }
    a.button:hover {
        background:url(http://i.imgur.com/LEmtZK3.png) no-repeat 0 0;
    }
</style>
<a href="http://www.facebook.com/" class="button"></a>