A WebmasterWorld thread is reporting a Google Image Search update of some kind.
It appears there was an image search index update. mssfldt reported in the forums the update started about mid-November. He keeps track of about 20 sites to track updates on images. We also have some other confirmations from other members.
The last Google image search update I covered was in June. He did say that there was an update in September that I missed.
Forum discussion at WebmasterWorld.