To reproduce: go to an image, rerun extraction (Yes and then refresh Now). Go back to Data page and manually refresh the page, and watch the thumbnail disappear. For some reason this doesn't happen to all images, so be sure to try more than one in case you get lucky the first time...the problem gets fixed on its own however after a while, I assume it's when the server restarts (If i stop my eclipse dev. machine and restart, the thumbnails show up again, problem is visible on mmdb-dev as well).