There Is a few complications changing this banner, it would be much easier if you actually provided your banner. Anyways
To change your banner go to Control Panel » Customize design » Common templates » Style sheet (CSS)
Find this line (located on 7th line)
Quote
#content {vertical-align: top; background: url('/.s/t/825/3.jpg') top no-repeat; padding-top: 199px;}
Replace blue url with your banners url