Add the current unixtime to the image file name like so:
The web server will interpret this as an encoded parameter and thrown it away when serving images. (If you are using parameters for your images, then I assume you know what you're doing. Maybe you're using an auto-thumbnailer or something)
Anyway, each HTML update will contain a slightly different image URL. defeating the (flawed) browser cache mechanism and cause the image to reload properly.
