I think the reference to this will be in stylesheet.css (in the templates/css folder of the template you are using).
Seems like you are using a custom template, becuase in a default install, the header background image is called header_bg.jpg .
In my case, the CSS looks like this:
Code:
#logoWrapper{
background-image: url(../images/header_bg.jpg);
background-repeat: repeat-x;
background-color: #ffffff;
height:75px;
}
Anyway, look for header_back.gif in your CSS file(s). Change it to a JPG of your choice, and then make sure the actual image is loaded into the images folder in your template's images folder.