I did this in probably a very messy (but works) kind of way just today.
Put in a Site Title (it shows in your browser tab) under the theme Options. If you don’t do this, none of it works.
Either change the style.css (or your child theme style.css if you’re doing that) like this:
#site-title {
padding:5px 0;
margin-left:-400px;
margin-right:auto;
border:0;
height:110px;
background:url(library/images/header5.gif);
background-repeat:no-repeat;
background-position:center;
The margin-left pushes your title off the screen (you may have to up the number if it leaks back on), and the background will obviously be your own image.
I think that’s all I did. If it doesn’t work I can look closer ??
HTH!