From SPCodex, The Smashing Pumpkins wiki
.mw-ui-button {
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;
height: 30px;
width: 30px;
vertical-align: -10px;
}