Over the past few days, you have may have noticed that thumbnail images weren't appearing. The reason is that the server we were using for generating and displaying the thumbnails crashed, and had to be moved to a different server. The new server seems to be working fine, and thumbnails are now being generated again.
Thumbnails for all old replays have been lost. However, I am in the process of regenerating all of the thumbnails, though this will take quite some time, since I only have one server working on this, and new thumbnails obviously have a higher priority. Rest assured, however, that all old thumbnails will eventually be restored. Sorry for any problems this may cause.