How can i change a picture every 20 minutes on a website?

-I want to change a picture every 20 minutes on my website so its diffrent every time. Does any one know how i can do this. I do not want to have to change it every 20 minutes my self.Depends on your website (like which one do you use for your website), also i don't think you can cause, you website would have to take random pictures from your computer to work.