Make a folder with the backgrounds you want displayed randomly. Open System Preferences, then "Desktop & Screen Saver". Click on the "Desktop" tab. Drag and drop your background folder into the sidebar there, highlight it, then click "Change picuture:" at the bottom. Set a time interval, then click the "Random order" checkbox next to it.
The time interval won't be random, but the selection of the picture will be (well, as random as a computer can generate).