A WebmasterWorld thread reports some changes to the rankings of Google images. One member reports;
Last Monday Google image search was giving me 32% of all referrals. Today it's 16%. So definitely a big change.
I also noticed some changes in my referrals from Google Image Search.
Some believe the changes are due to Google being better able to categorize images with Google Image Labeler but it seems too new to me to be already a piece of the algorithm.
Forum discussion at WebmasterWorld.