Its maybe a little late but here are the steps:
1. Create a new site or edit your existing site
2. Scroll down and on the right side is a box called Featured Image.
3. Click on “Set featured image” and upload a image you like or choose one you already uploaded to your Media Library.
4. Choose an image and click on show
5. Click on “Use as featured image” and save the page
6. That’s it. View the page and enjoy your different page header image!
