I did this;
- Created a folder on the server called ‘headers’
- Into that, I uploaded several different images that would suit as a header image.
- I saved the script above as a php file (call it what you will - mine is ‘rotate.php’) and uploaded into the same ‘headers’ folder.

  	 Now I edited the CSS file.
In WP, there is a line in #header which can be used to specify a url for the image to be loaded. This needs a light alteration:

  	 Mine was
#header {
background-image: url(image.jpg)

  	 and it is now

  	 background-image: url(headers/rotate.php);

  	Now, when the CSS is called, the background image is chosen randomly by the script.