This article originally appeared in: https://support.apple.com/en-us/HT204414
If you have multiple photo libraries on your Mac, you can choose one to be the System Photo Library. You can access images contained in the System Photo Library in apps like Pages and Keynote, and set them as desktop pictures and screen savers in System Preferences.
Photos in your System Photo Library are available in apps like iMovie, Pages, and Keynote. You can also sync them to iOS devices and Apple TV via iTunes. And if you want to use your own images as your desktop picture or screen saver, the images need to be in your System Photo Library before you can select them in System Preferences.
If you have only one photo library, then it's the System Photo Library. Otherwise, the first photo library that you create or open in Photos will become the System Photo Library. If you have more than one library, you might need to designate a System Photo Library, so other apps can access the photos and videos you want them to use.
Follow these steps to designate a System Photo Library:
- Quit Photos.
- Hold down the Option key, then open Photos. One of the photo libraries is already designated as YourLibraryName (System Photo Library).
- Choose the library you want to designate as the System Photo Library.
- After Photos opens the library, choose Photos > Preferences from the menu bar.
- Click the General tab.
- Click the Use as System Photo Library button.
If you open a second or different library in the Photos app, and you haven't designated it as the System Photo Library, other applications will use photos from the original System Photo Library. Hold down the Option key when you open Photos to see which library is set as the System Photo Library.
iCloud and the System Photo Library
You can use iCloud Photo Library, iCloud Photo Sharing, and My Photo Stream only with the System Photo Library. If you choose a different library in Photos without designating it as the System Photo Library, the iCloud tab in Photos preferences is disabled:
If you designate a new library as the System Photo Library and then turn on iCloud Photo Library, the photos and videos in the new library will merge with those already in your iCloud Photo Library. If you want to keep the contents of your photo libraries separate, don’t turn on iCloud Photo Library for more than one library in Photos.