First find the banner image on the site by one of the following methods from the Admin Menu:
Site Content and Layout
Search all.PHP
allClasses
After getting to the banner using one of the methods above click Edit
If you want to change where the banner is linking, you can edit the Link
If you want to edit the image, you can upload a new file and replace the existing banner image
This is useful for example if the newsletter tool might want to have a different banner from the homepage, or you want multiple banners for different newsletters, or you want to change the banner on the homepage from time to time.