Upload an Image from Images and Logos in the backend - and set the Image use to 'Banner'.
View the front of your event website.
The horizontal navigation will also allow you to implement a banner if you choose to. All you need to do in order to display a banner is change the “Image Use” to banner in the Images & Logos section and make sure your template is horizontal navigation.
Select the Horizontal Template from the Front of the Event Site.
This is how the banner will look on your site. To make the most of this feature, make sure that the image is quite a bit longer than it is wide.