Hold it there!
It has nothing to do with IIS. It's called "browser caching" and browser does it automaticaly. You can even instruct it to do it every time or never (in "options"), or control it server-side.
For example, if you add (
VB-code in this case, for IIS):
Response.CacheControl = "no-cache,post-check=0,pre-check=0"
Response.AddHeader "Pragma", "no-cache"
Response.Expires = -1
...to your headers all content to be delivered to browse will be reloaded from server every time you look at it (no cache). Otherwise it's up to your browser to decide when to reload pictures.
So it has nothing to do with web-server. M$-bashing is pretty popular "sport", but in this case it's not a culprit.