//HTML
<a href="https://www.facebook.com/" id='facebook-button"></a>
 
 
//CSS
#facebook-button a{
    background-image: url("http://i.imgur.com/VBam6AY.png");
    background-color: transparent;
    padding: 0px 0px 0px;
}
 
#facebook-button a:hover{
    background-image: url("http://i.imgur.com/LEmtZK3.png");
    background-color: transparent;
    padding: 0px 0px 0px;
 
}