Are you wanting a random image on page load or do you want something to set up a pic of the day?

You can use a banner rotator script if it's a random image on page load, you just have to be sure to edit the pic size to be displayed and that your images are all uniform in size as well.

Or take a look at this for a pic of the day script

http://www.scriptz.com/details/900.html

or here for a free one

http://www.schlabo.com/info/potd/2.01/index.shtml