Symptoms
To change the banner I opened site in Sitebuilder Wizard and change to page Design. Then in section "Banners" on the right clicked button "Browse...", located new banner and then clicked button "Upload".
However new banner is not shown in the list of banners on page "Design" and in site preview. Why?
Cause
Old banner has been cashed in your web browser and new banner is not shown even it has been uploaded correctly.
Resolution
To see new banner that was uploaded on page "Design" you need to clear cache in your browser and reload the page. To reload the page with clean cache press button "Shift" in your keyboard, click right button on your mouce and press "Reload".