Most of these images were brought to the temple from the river as explained by the priest. They were thrown and he and his father picked them up from the river and installed them. However this claim need not be taken on the face value. Whenever new shrines were erected in Gaya, priests tended to collect sculptures from different places and install them within their shrines. This may have been the case at this shrine as well. -- Also the name of the shrine as Prapitamahesvara seems to be a smart strategy to use a pre-existing familiar name to build a reputation/following for the shrine.