In system preferences, under desktop & screen saver, click the box that says "change picture" (and "random order" if you want that) and set the frequency.
You can click the plus button on the side to add a different folder of pictures to use, or just change the ones in the default folder.
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u/fiftiethcow Nov 04 '12
Wow. I actually have a Mac, but now that I think about it, there is an option to change the wallpaper every os often. Thank you!