I agree , it would lead people who dont take the site seriously to add strange images, even though personalizing things would be good, i doubt it would happen.
They want originality, and they achieve that with the armatars. Plus, mods already have enough to do on the site, they don't need to be searching people's profiles all day looking for obscene pictures. This has been covered many times before, and AG is sticking with the armatars.
First, controlling over personal armatars would be hard. Also, accepting or declining one would be very subjective and people wouldn't be very satisfied. Second, armatars give site some kind of atmosphere, which is not very important, but still good.