Access the Admin Menu, then press the Course Management subitem in the E-Learning section. Find the course in the list on the main Course Management table and click on the course description. While in the Properties tab, move to the Player section then select Player Background Image.
From this tab, you can manage the course player, which dictates how the course training material is visually presented to learners
In this section, flag enable a custom background image for the course player of this course, then select it among the available images (Select Image option) or press the Upload Image button to upload an image from your device. Set now whether you want to keep the original image size or stretch the image to the default page size (900 x 510px).
Always click Save Changes.