You need to have the header either stored on the site (website directory)->images->headers or elsewhere online. If elsewhere online you need to know the url to point to it. The size of the image (if full screen) should be 940 by 180 pixels, resolution 72. The header image can be either jpg or png.
To add a logo or a header file to the site go to extension->template->(default template)
In the template manager select the options tab. Make sure the advanced option it open. Go to logo select. This will take you to the folder for images->header or you can enter the url. Select the header image. The static option for fluid layout seems to give the best appearance.