It appears they are classifying sites as porn and non-porn now. If your site is classified as porn it will only be shown in the image search if it contains a "whitelisted porn term".
The problem with this is that it means less traffic for sites classified as porn by Google. There will also be some terms that are clearly porn related but that Google does not yet have whitelisted as a porn term.
I've long suspected that they have begun doing this for some regular search terms (sites classified as porn are no longer shown in the top results) and the day will probably come where they do it on all terms as they are now with image search.
I have a theory that part of the boost sites like tumblr, dailymotion, and youtube are seeing in the results for some terms is partially due to the domains not getting classified as porn according to Google while our sites are. When in doubt Google gives the non-porn classified sites a boost.
Just some theories and speculation. The advice is only worth what you paid for it.
