It's the black, apple-shaped icon in the upper-left of your screen.;
,, It's near the top left of the Preferences window.
 It's at the top of the dialog box.
 Make sure it's blue and a checkmark is displayed.


, You can choose to have the desktop picture change every 5 seconds, every minute, every 5 minutes, every 15 minutes, every 30 minutes, every hour or every day. You've now changed the frequency of your Mac's desktop picture changes.


You can also choose to have the picture change upon log-in or when the Mac wakes up from sleep.
Click on the Apple menu.
Click on System Preferences.

,
Click on Desktop & Screen Saver.
Click on Desktop.
Click on the checkbox next to "Change picture.",
Click on the drop-down menu next to "Change picture.",
Click on a frequency.
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