1. Click the apple icon on the Menu bar.
  2. Select System Preferences in the drop-down menu.
  3. In the System Preferences window, select Desktop & Screen Saver. 
  4. In the window that opens, go to the Screen Saver tab.
  5. In the left pane, select a screensaver style.
  6. In the right pane, open the Source drop-down list.
  7. In the Source list, select the source of the screensaver images. In this example, I have selected the Photo Library. You can select from macOS presets or your own photos.
  8. Click Choose to complete the process.