see Background Changer V1.2 in Buttons/Scripts.
minimum DO version 13.10 required
notable changes
- new Global Opacity setting
- Static/Dynamic lists - means the images can be read into a list one-time or every time.
- OnPeriodicTimer() Event used for timing, means script is not always running
enjoy 
An interesting usage is setting up a FOLDERCONTENT button for your latest photo shoots. Go and do some raw burst shots, transfer the images and start the slideshow. Set up a horkey ro delete current image and set slideshow to something like 1 second. You can then cull your burst shots.
One thing to note - 20 thousand images takes about 10 seconds to compile a list of paths, if dynamic mode is on that means the background will change every 10 seconds. If static mode is on then only the first image will take 10 seconds and subsequent images 1 second.