frontcube.com
Please see the show notes to fix the gotcha addressed in the screencast.
The idea is that I click the bookmarklet on the page on which I want the images and styles to watched for changes, and the page starts pinging a local php resource (running on apache) to check for changes. When the php detects a change, that particular resource (css or img or swf) is reloaded, instead of refreshing the entire page.
This setup didn't require an additional server running, neither a browser extension, but is limited to apache and php, which was good enough for me at the time.
PS. Now I moved on from apache+php, if anyone is interested in this, the code is at https://bitbucket.org/sharat87/pollfresh/src/tip/pollfresh.p... is just a php file, put it in your webroot and open it through apache. I don't know if it should still work, but one can easily get it to run.