Have you checked the Event Logs to see if you see any errors there? How about the local system logs?
I would try going to System in admin and clearing your cache and restarting the application. If that does not work, try resetting IIS or restarting the system. Could be there is a hang up some where.
Since images are stored on the file system, you could also look to see there is not a local file permission issue that could be causing the images not to load up.