You can add a background image for your iPad Kiosk:
On your home screen, click the Kiosk you would like to edit
On the sidebar, click Background
Click Upload to add one or multiple background images
If you choose to have multiple, choose the Image Transition style between Fade and Slide
If you want to have a Screen Saver, click Enable Screen Saver
Choose the text that will be displayed on your screen saver screen
Click Done
Click Save
Click the upload icon to Push to Kiosk
Background images should be the native resolution of the iPad screen. For recent model iPad's such as the iPad 6th Generation released in 2018, the best resolutions is:
Landscape 2048 x 1536 pixels
Portrait 1536 x 2048 pixels
File size less then 2MB