If you prefer a background image to a header image on your experience, you can set that up by starting on the "Style" tab within the setup of the experience. From there, click on the experience theme at the top of the menu that opens. Lastly, at the bottom of the theme settings, you can upload an Experience Background Image.
Note: If you would like to browse our stock images and gifs, or upload an image previously added on your account, you can click on the "Select An Asset" text above the file submission box to open those options.