At the time of Adi Shankara apparently people were worshiping all sort of gods and goddesses. Once of the changes that he made was to simplify things and members of that tradition worship only five forms of the divine on a daily basis. This is not to say that they ignore all the others, but these five are central to their tradition. Strictly speaking they would use 5 stones from various parts of india. The lack of form is a reminder that the divine is beyond form, but most people have some small murtis mixed with the stones.
In the center is the Shiva lingam. On the right is a small statue of Annapurni, the goddess who feeds everyone. This is the family deity of Seetharam and is used instead of another form of the goddess. The dark stone at the bottom is a shaligram, a rock containing a fossil which is used to represent Vishnu. On the left is a stone that represents the Sun. On top, the red corral represents Ganesha.