Actually, I have a PHP script that is called as if were an image with another forum. I like this idea, but question whether or not it should be a feature or a mod. Honestly, I think if it were a feature, you would see a lot more people use it.
You can use this script as your background-image, just upload all images in one directory and upload this script then add the link to the script to the background-image section. http://ma.tt/scripts/randomimage/
It's really easy to do you just point your header image at a script on your server and it displays the images, no need for this to be a built in feature imo when the php scripts to do it easily already exist, it's a less than 2 min job to set up