There are almost unlimited choices for this type of thing!
You can use really simple javascript programs or feature filled perl or php scripts…..
All of this depends on whether you are simply rotating images (pics) or whether you want an ad banner or a banner which is clickable - each banner goes to a different site.
I, for instance, use an ancient perl program, WebAdverts, which still works fine…that is for managing lots of banners.
If you just needed 4 rotating images, you’d probably be better with a simple javascript program. With either type, something can be worked out to allow the client to change things…if they have basic knowledge.
We should mention that none of this has anything to do with EE. This would be the same for any static page or CMS.
Here is an example. If I have a client that has 2 banners running in a certain position, and I want them to be able to change them - I “serve” them from the clients site! For instance, if the images are
banner1.jpg
banner2.jpg
as long as the client replaces them with the same file name (different images), then they will work, since they are always fetched from the clients server. Sometimes I actually create a user on my machine so the client can then ftp them to my site.
So, let google be your guide and fiddle with the various programs outside of EE…then when you find what you want, perfect it and put in EE in various ways (global, etc.)