Yes, you can add as many images to the header as you want, a background image too (background images can be set easily from the Styles Manager).  For slideshows/galleries, you can do that within the page section, here's the user manual topic on it: https://www.ultimatewb.com/photos-app (scroll down to Adding Photos / Slideshows / Photo Galleries To Page Photo Section - https://www.ultimatewb.com/photos-app/add-photos-slideshows-galleries )

It can't be added to the header because of the setup...if you did want to go there on a page, you could take off the header for that page (an option towards the middle of the add/edit page), and then that will move up the slideshow content to the top.