I just wanted to know - how do you get different pictures to show up each time you enter a page?

For example, on the front page of Lissa, it shows one picture, then it shows a different picture when you next go on the page.

There are these 2:

http://www.texaswebdevelopers.com/examples/randomimageonrefresh.asp

http://www.dmxzone.com/ShowDetail.asp?NewsId=5427

Thanks