Its common Hindu mythological symbol, that the AnanthanShayana (a Serpent Ananthan make shield with its hood to Lord Vishnu, another occurrence, Krishna when he was a kid, was shielded by A Snake from the rain. ). But in the picture, you can see a different canon. Its Lord Budha being shielded by Naga(Serpent ) Mucalinda.
well, its folklore stories , we can say but I am trying make rational thoughts about this symbol. I can only consider Hindu version as mythical folklore only. But coming to Buddhist version, we can make some rational assumptions and interpretations. Its Buddha and Naga. Buddha is being protected by a Naga. Dr. B. R Ambedkar explained about his choice of Nagpur as the venue for the conversion to Buddhism, he clearly mentioned that Naga race were Buddhist people. Come, again, Buddha is being protected by Naga, means Buddha being protected by Naga people. It was recorded history that Buddha had to face so many assassination attempts at his life time. So its a truth that Budha was protected by Naga race depicting by this symbol.
So this symbol must be originated by Buddhist culture, later only Hinduism copy-catted it.
References : –http://en.wikipedia.org/wiki/Mucalinda