I do this here http://www.ecol.org.uk (hit refresh to see different header images)

what you need is a file called rotator.php which you can get free here:

http://automaticlabs.com/products/rotator

drop the rotator.php into the folder same folder as your images and then call in up from the skin CSS like this

div.pageHeader {

	padding: 0px;
	margin: 0px 0px 0px 0px;
	height: 120px;
	background: url(img/rotator.php) no-repeat #FFFFFF;
	background-position:left;
	}

in this instance my images are in www.whatever.com/img

you can call up random images using HTML like this:

<img border="0" src="http://www.yourdomain.com/yourdirectory/rotator.php"  />

As far as I recall I did no config on the rotator.php so long as it sits in teh same directory as the images but there are instructions in the php file and on teh website

thanks for the help, that seems to be exactly what i'm looking for