If you are tired using the same images on your screensaver again and again or you simply want to personalize it, you can use your own photos from your iPhoto library. All you have to do is flag the pictures or images you want to appear as your screensaver and unflag those which you don't want to include. This step-by-step will tell you how to use your iPhoto library as a screensaver.
1. Go to your Mac menu bar and click the Apple icon to go to 'System Preferences.' Then click the 'Desktop & Screensaver' icon on the menu bar. Locate the 'Screensaver' tab on top of the screen and click it.

2. On the 'Screensaver' page, you will see different types of animated effects for your screensaver on the left-side panel. You can present your slides in random order, cross-fade which allows images to transition smoothly in succession with each other, zoom an image back and forth to show further details, or keep your photos at the center.

3. After choosing your effects, go to the right-side panel of the screen and click 'Source.' A drop-down list will appear, click 'Photo Library' and flag the images you want to include on your screensaver. If you have more than a hundred photos in your library, you can also use it as a mosaic display.

MacOS Big Sur 11.3 includes a hidden 'Hello' screen saver that appears to be designed for the new iMac models, but which can also be installed on any Mac that's running the 11.3 update. Launch the Photos app from the Launchpad on your Mac. A prompt will appear asking you to select the library you want to access. From this list, choose your iPhoto library and click on Choose Library. Your iPhoto library along with all of your photos will launch in the Photos app. You can now manage your photos in this app from. The Mac Photos app works well for most users, but occasionally a variety of hiccups can be encountered when interacting with photo libraries, ranging from crashes, to hanging on Photos app launch, a failed import, thumbnails missing from an image library, pictures missing from a library that was imported, or even a completely empty Photos app.

Photos in your System Photo Library are available in apps like iMovie, Pages, and Keynote. You can also sync them to iOS devices and view them on Apple TV. 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.

Mac Screensaver Photo Library Missing Words

Follow these steps to designate a System Photo Library:

Quit Photos.
Hold down the Option key and 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 Photos, Shared Albums, 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:

Mac Screensaver Photo Library Missing Files

Mac Screensaver Photo Library Missing

If you designate a new library as the System Photo Library and then turn on iCloud Photos, the photos and videos in the new library will merge with those already in your iCloud Photos. If you want to keep the contents of your photo libraries separate, don’t turn on iCloud Photos for more than one library in Photos.