To set any page banner Go to Customization -> Theme Settings -> Breadcrumb -> you will get below option settings.

Background Image:-

First option is to set background image for all the pages, which will be appear where no image is assigned to any static page.

Page Background Image:-

This Image will be appear as banner of all static pages like About Us, Contact Us, FAQ etc...

Note: For Collection Page Banner:-

At the time of collection creation, if the collection image is assigned then that image will be displayed else collection's first product's image will be displayed.