Each page and post has a header where you can set your featured image and add some description about the page/post.
To enter the desciption, look for "Add Page Description" field below you wordpress post editor where you edit post or pages.
Banner Image : You can upload your banner image like you upload featured images. Banner image option is right below the featured image option next to the text editor. If the banner image is less than 1408px in width they will stretch to cover the total width of the header.