Whenever there is an image search change at any search engine, Zeus, a WebmasterWorld member, knows about it. He posted a thread at WebmasterWorld noting that he has seen a change to the moderate filter used by Google Image search.

Zeus said the change is a big improvement to what was there previously. Specifically, images often wrongly associated with being adult pictures are no longer classified as such. He said, and I quote:

WebmasterWorld administrator, Tedster, also noted that Google Image Search’s product manager, Peter Linsley was interviewed by Eric Enge last week. There are some good tidbits in there.

So if you noticed an spike or decline in Google image search referrals, this may be why.

Forum discussion at WebmasterWorld.