In Design tab choose Header -> Banner. You will be able to change Banner image as you want by clicking Add Header Image.
You can choose Banner from our Library or upload your own image. Please be noted that the resolution that suites for Banner is 1440px x 520px.
You can also set the Transition Effects for baner.
Also set for Header Space/Background Color/Header Width Header Space is an individual section at the top of your page which you can add any color (but you can't insert an image).