How to insert logo into header using CSS only

In Hiero, the header is created solely by the CSS using a background colour and a font. How do I insert a logo to cover the letters in the stylesheet. I ‘ve seen it explained for use in the header.php but could someone please give me some guidance for the CSS. Here is the header code in the CSS:

/* HEADER
———————————————– */
.site-header {
padding: 20px 0 0;
background: #16171b;
}
.site-branding {
float: left;
}
.site-title {
margin: 0;
font-size: 4.25em;
font-family: “Yanone Kaffeesatz”, Arial, Helvetica, sans-serif;
text-transform: uppercase;
line-height: 1;
}
.site-title a {
color: #fff;
text-decoration: none;
}
.site-description {
color: #999;
font-family: “Yanone Kaffeesatz”, Arial, Helvetica, sans-serif;
text-transform: uppercase;
letter-spacing: 1px;
}

/* MENU
———————————————– */

Any help would be greatly appreciated.