If you are like me and use multiple monitors, you can quickly, and easily I might add, make the wallpaper on all monitors the same.
Open “System Preferences” and go into “Desktop & Screen Saver”. A window will show up on each monitor. Move them closer to each other for more ease. Click and drag the preview of the wallpaper from one monitor to the other(s) wallpaper preview box. Bada Boom. Bada Bing. Done. See the amazing GIF at the top for the visual.