Max image size 5MB, (due to browser local storage limits).
Take care with large files, they may impact performance.
If adding more than one URL separate them by spaces or on new lines. If more than one is entered a random background image will be used on load.
Unsplash can be used for random images, eg:
https://source.unsplash.com/random/1920x1080/?night,day,sky
Change parameters after .../random/ for more options. Loading times may vary.