Just opening another machine and connected to edugeek to find a lovely April fools day banner, which hadnt appeared on my office PC.
Clearing the cache in google chrome reveals this new banner.
It seems a shame that people who use caching browsers will miss some of the banner artwork you put up.
I had a similar problem with a web cam image on a portal page which was solved by appending "?random=timestamp" to the image source, obviously getting users to download a new image every time they load the page would hog bandwidth and defeat the object of caching. But having PHP generate the timestamp for the hour so the image is only downloaded once an hour might work.
could just use ?version=<number> and then just increment the number on each update, and thus only clearing the cache when ever there is a update.