MAC:
Choose your screen size and a full-size wallpaper will open, right-click (or hold control and click on it) and select "Save Image to the Desktop". Drag this newly added file onto your current wallpaper seen in the Desktop & Screensaver section of the System Preferences utility found under the Apple menu.
Windows PC:
Choose your screen size and a full-size wallpaper will open, right-click on it and select "Set As Wallpaper".
