Thanks, speedshopping. That's a very useful Google Help Forum thread - so let's put the quote here:
on may 6, google analytics changed the way they report accesses from google image search. in "traffic overview" they used to be seen from images.google.com (referral), now most of them are included in "google.com (referal)".
so even if google analytics seems to show the referals from image search dropping to 0 since may 6, that's in fact no the case at all.