Miscellaneous technical suggestions.
Here's a way to get an image chosen at random from a directory of images on a web server.
With this, you can change what's being displayed simply by adding images to or removing them from the directory. It uses PHP, so you'll need to have that enabled on your web server if it isn't already. It's a very simple script that just picks a random file (one with an image extension) from its own directory and returns it.
Here's the script: index-php.php.txt.