To change the background image:

Go to Style > Choose the Background tab



Tip:
You can use the same background you are using in the registration page or choose another one. In the case of tablet screens, when running the exercise, the rest of the icons for the other channels will appear at the left of the screen so, if you are using a busy background, it might be a good idea to add a white bar to be able to see them. 
This is an example: