You probably won't see "header_bg.jpg" in the source code as it's a STYLESHEET call. The div ID is logoWrapper, and that divID also applies to logo.jpg (or logo.gif). So the path to logo.jpg will also be the path to header_bg. Just make sure header_bg is in the same folder.
If you look at stylesheet.css, you'll probably see:-
PHP Code:
#logoWrapper{
background-image: url(../images/header_bg.jpg);
background-repeat: repeat-x;
background-color: #ffffff;
height:75px;
}
... or similar.
That is the CSS reference for any div ID in your sourcecode for logoWrapper.