How can I configure Apache so clients cache images but still check for updates?

I'd like to set up an Apache vhost such that browsers will cache images locally but will check back against the server for updates each time prior to displaying images.

I have this currently:

<VirtualHost *:80>
    ServerName cdn.localhost
    DocumentRoot /Users/chad/development/support/cdn

    FileETag None
    <Directory /Users/chad/development/support/cdn>
        AllowOverride None
        Order Deny,Allow
    </Directory>
</VirtualHost>

and, at least locally, files are cached indefinitely in browsers until I manually clear the browsers' cache. I don't like this. When I update a new image to replace an old image, browsers do not see this update.

I know I can turn caching off like this

FileETag None
<Directory /Users/chad/development/support/cdn>
    AllowOverride None
    Order Deny,Allow

    Header set Access-Control-Allow-Origin "*"
    Header set Access-Control-Allow-Headers "Content-Type"

    Header unset ETag
    Header set Cache-Control "max-age=0, no-cache, no-store, must-revalidate, proxy-revalidate, no-transform"
    Header set Pragma "no-cache"
    Header set Expires "Wed, 11 Jan 1984 05:00:00 GMT"
</Directory>

but I don't want it completely off.

So, how can I instruct browsers to cache images but still check against the server for updates?