In your css is the declaration for header
border-bottom: double 3px #aba;
border-left: solid 1px #9a9;
border-right: solid 1px #565;
border-top: solid 1px #9a9;
font: normal 230% 'Lucida sans Unicode', Verdana, sans-serif;
padding: 15px 10px 145px 120px;
in the 'background-image' specify the location for your image.
If it was called "bananas.jpg" for instance, and you put that image in your WP directory, the line would look like this:
Change other variables in that decl one at a time, and check the changes each time.