Earlier this month a friend, Amy Crehore, was telling me she was getting traffic from her images, Google Image Search traffic, that she never had before. I mentioned that Google probably changed something - as she didn't change anything on her site(s).
Now we know what was changed - Google Starts to Index Images Uploaded to Blogger - in a post on Google Operating System blog:
Even if this sounds hard to believe, Google Image Search started to index images uploaded to Blogger in December 2007. Until this month, all the images were prevented from being indexed by search engines for unknown reasons. This move is closely related to the fact that the images hosted at Picasa Web Albums started to be indexed by Google.
The chart from Google Analytics shows the number of referrals from images.google.com for this blog:

I haven't noticed anything in particular on my blogs - but I haven't looked closely either.
