From SPCodex, The Smashing Pumpkins wiki
.ultimateguitar .mw-ui-button::before {
background: url(https://upload.wikimedia.org/wikipedia/commons/thumb/b/bf/Ultimate-Guitar.com_logo.png/30px-Ultimate-Guitar.com_logo.png);
content: '';
display: inline-block;
height: 30px;
margin-right: 6px;
width: 30px;
vertical-align: -10px;
}