The best way I've found to do this is to repurpose an already-existing link on the page with a background image. It takes a little extra work. Here is a an example that works with the demo MistyLook theme. It uses the site title to ge the link, so note that you must have a site title set before it will work on your blog.
#header {
width: 760px;
padding-top: 30px;
}
#feedarea {
position: absolute;
right: 0;
}
#header h2 {
display: none;
}
#header h1 {
text-align: left;
text-indent: -9999px;
}
#header h1 a {
margin-top: 10px;
display: block;
width: 760px;
height: 190px;
background: url('http://burnerprep.files.wordpress.com/2012/11/cropped-header-bg.png') center no-repeat;
}
#headerimage {
display: none;
}
#content {
clear: both;
}
Note that you may need to make additional adjustments.