The home menu background image appears behind the messages box and the home menu buttons on the home menu screen. On the page Pages & Structure, tab Home, you can see a preview of your screen.
To add a background, click Add image at the bottom of this preview. When you have already added an image before, this text is Change image.
A screen opens where you can select your images. There are four device types (mid-range phone, high-end phone, mid-range tablet, high-end tablet), for which you need to upload both a landscape and portrait version. Click a grey button to select the image for that type.
The images sizes are specified in the Artwork Guidelines below Home menu background. Using the suffixes as listed in this document will allow for all of the images to be uploaded at once through the button Select multiple.
After saving your images, the preview is automatically updated.