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