Recently I have noticed this phenomenon , that user upload pornographic pictures to there profile on the stackoverflow site,
who to prevent this ?
I guess you would have to report offending Gravatars to Gravatar directly. I expect they do some form of basic content filtering anyway .
If you ask me, though, none of these images can be classified as even remotely pornographic by most today's standards. Advertising on any western TV station or magazine will show more arousing imagery.