Refreshing a document images

My site generates chart gifs on the fly and embeds them in HTML documents (using ASP pages). The problem is that users don't see updated charts unless they click on they refresh button on every page containing a chart. Other solution is to disable the browser's caching.
Both of the solutions are not good enough since they require manual intervension and generate extra load on the server since each page containing also lots of static images that doesn't have to be loaded again and again from the server.

Any other ideas ???

thanks,
Avi.