Tap Controller to Start Display

Does anyone know how to change the background image of the screen that says "Tap Controller to Start?" The default is a shot of the Golden Gate bridge in the clouds. We've manage to change the display for Digital Signage but when there's a call starting, it changes to this image, which we'd like to change. Can't figure it out!

here is an image: https://postimg.cc/TKzXmcxn  

Thanks in advance!