I used the image randomizing PHP script here (http://babytest.livejournal.com/9923.html) -- it's pretty easy to set up as long as you have webspace that accepts php; you don't have to know php at all.

But all the coding is at offsite webspace -if you don't have webspace that can run scripts, I don't know if there's any way to do it. (My webspace went down, which is why my header images are broken right now.)