If you are using the OpenWater default theme, you have the option to upload a banner. The recommended banner size is 1050px wide and 250px high.
This is an example of a nice banner: https://mdaiaawards.secure-platform.com/a
To upload a banner, click Public Settings and then Website in the left side navigation.
Tab over to General and then upload your banner under the banner section.
Check your banner by clicking Public Website at the very top of the page.