Generrally, just tag a random number onto the end of your URL, like:
"http://www.myserver.com/image.jpg?r="+rnd(3000).tostr()
should force a new image download.
then perhaps run the purge function ever few minutes.
Screenshades: The first Screensaver for Roku2!
Musiclouds: The best free internet music, on your Roku!
Ouroborialis: Psychedelic Screensaver for Roku!