Into something customized for your SharePoint install is to swap out the image used in the header style sheet class. You can easily change the graphic in the OWS.css style sheet located in the \60\TEMPLATE\LAYOUTS\1033\STYLES directory or add it to your custom created style sheet that is specified in the site settings. These changes will affect both portal and WSS sites.
Here are the steps => by Heater Solomon a nice CSS :
.ms-bannerframe .ms-banner ....
EROL MVP SPS