The rotating images and content on the Intranet home page are brought in with the Zen Random Content macro. You can learn more about that macro here, but essentially, the macro animates content from a set of child pages.
You can swap in your own images and text by editing the children pages used in Zen Intranet: Intranet Banner One, Intranet Banner Two, and Intranet Banner Three (You can do a search on those to find them, or search on the parent page: Intranet Banner Content.)
The large banner images are brought in as background images to a page's section. To swap in your own image, simply attach your new image to a page, then indicate it as a background image in the Section Designer. (As a guideline, the images attached for Zen Intranet are 1125×450 pixels.)
Click the edit pencil in each section to edit the text and Zen Buttons. (We are using CSS to target those headers, increasing their size and changing the default padding and margin.)