OT: How to auto-rotate images in a web page window

[joe at actionline.com]

Recently, I have noticed what seems to be a new (and very nice) feature of
gradually rotating images in a window on many web pages, but I can't find
out how it is done. Here are a couple of examples:

http://www.monkeytreephoto.com/index.php (very slow starting)
http://www.filmmakeraccessories.com (six second delay between images)

What do I need to get something like this to work on my web pages?

I'd like to speed it up a bit and get it to cycle through a variety of
images as they are added to a folder starting with the most recently
added.