There are many ways to change wallpaper automatically in Linux, here’s a very simple and fast one: by using a program called Variety.
Available in all repositories of the most common distributions under the name variety, it is very easy to use compared to other methods.
It inserts an indicator in GNOME:
And there are a lot of options available:
Download from the internet, from a subreddit, take it from a directory, make specific filters (ex: always blur, or make grayscale), add a clock or a motivational phrase.
Or: choose only light backgrounds, or only dark backgrounds, or have a shade of color to your liking!
Finally: don’t like the background that was randomly chosen? Just give a “roll” with the mouse wheel over the indicator icon and it will go to the next one!