Adding the cache-control header doesn't help because it's probably the images that are being cached, not the HTML page. And the header only affects the object that is being transfered with it (in this case the HTML), not the objects that are referenced in that document.

Open one of the images in a tab (or window) of its own. Then request the HTML page from your server, and then reload the image separately (in Firefox with Shift+Control+R) . Does the image get updated?