I was playing with RRD and network graphs at home (what do normal people do on holidays?). Anyway... If you include this script in your markup, it will periodically reload all images it can find. For better or worse...

reload-images.js

(function() {
        function updateImages(){
                var images = document.images;
                for (var i=0; i<images.length; i++) {
                        var imagename = images[i].src.match(/((.*)\.(png|gif|jpg))/i);
                        if (imagename) {
                                var timestamp = new Date().getTime();
                                images[i].src = imagename[0] + "?nocache=" + timestamp;
                        }
                }
        }

        setInterval(function() {
                updateImages();
        }, 300000); // 5 min

}())