Well this would assume that you know what CSS sheet the Editor is using and where the default header image is located. Which I don't. Can you give me a location for the CSS the editor is using? Much appreciated! 
If you right-click any (blank) bit of a web page (including the CMS Admin pages, you can view the source code which shows you (in the Head section) what css files are being referenced. When you know where the stylesheets are, you can view those too and follow the links to referenced images.
