Caching is a great way to improve performance on the Web;
however, sometimes you might want to force images to be
downloaded on each and every visit to a Web page. An example
of this might be a page that generates images dynamically
(for example, bar charts) but where the page uses the same
filenames over and over. Obviously, in this case, you must
require that the image be downloaded every time. So, to
prevent images in a directory from being cached, launch the
Internet Services Manager, navigate to the images directory in
question, then right-click the folder, and choose Properties.
In the Properties dialog box, select HTTP Headers, and then
click Add. Enter the following three headers:

cache-control:no-cache
pragma:no-cache
expires:0

This will prevent any images in the folder from being cached
on the client, so that you can modify them as often as you
like without fear of users seeing out-of-date versions.