If you want to change the look of a particular page and include a header, click next to the icon of the page in question in the Content panel, the Options button:
then click on the Header tab from the top and upload your images:
You can upload up to 3 headers.
You can also insert images from the web or text, by clicking on the appropriate tabs.
Recommended sizes for min. width: 640px for Portrait Header / 1024px for Landscape Header
The header design is important to evidence the exclusive concept relevant to your business or the idea you want to point out in your app. Make shure your header image is not too big so that your app wont be slow in download, and that the image is not too detailed because it might change between device and device.