You can change the desktop wallpaper for all users on Mac. Follow the simple steps below

Right click on the main desktop screen

Click personalise.

Then click desktop background.

Then click browse.

Choose a picture, (if there isn’t any go onto the internet).

If there isn’t any pics, go on the Internet & google images.

Type in something, E.G, jonas brothers. Choose a pic, click on it.

It should go bigger, right click, save picture. Save it in pictures.

Get the pic, click OK.