This image is used as the background image for the homepage of the web application. Choose an image with the highest resolution possible so it looks good on all screens. Also avoid using images with lots of different colors as it will distract from the main focus of the page, which should be the user's lessons. Images with a gray or color overlay are better.
Your image will look like this before the page fully loads:
Your image will look like this after the page has loaded:
Note that the background image you provide will not be used in the mobile apps.