This time of year I have replaced our site logo with a Christmas one. I have replaced it in the correct spot and purged the site cache in ACP.
A new visitor to the site is shown the updated logo. A previous visitor will see the old logo until such time they manually refresh their browser with Ctrl-F5 or whatever is the correct keys for that browser flavour cache refresh.
So question is, how do I force a refresh of the users caches or force their browsers to pull the update logo? And no, not using any CDNs. However do have a feeling that also some ISPs may be caching too.