You can add a Featured Content Slider to the top of any of your pages using the Featured Content Slider options found under the Settings tab for any page. Check out NationBuilder's documentation on using Featured Contend Sliders here: nationbuilder.com/configure_featured_content_sliders
The recommended image dimensions for featured content sliders on your website are 1300px wide by 450px tall.
Featured content sliders can also act as a banner/hero image for any page. Simply add only a single featured content slider to the page in question, and make sure that the current page slug into the Page to feature* field (i.e. if I'm adding a banner image to the Home page, then I would enter home into the Page to feature* field).