To use custom background on your mobile page, click on the image in the page properties:
You can use backgrounds from image library or upload any custom image for it.
Uploading images for the background, you should consider how the page will look like then. You can use a small image like this one:
By repeating it creates a beautiful pattern:
Or you can use a big image to fit a desktop’s screen. But the uploaded image must be less than 500kb.