If you are looking for ancient art in Polonnaruwa I suggest you take a drive to the Tivanka Pilimage or the Tivanka Image house. or the Northern Temple of H.C.P.Bell.
It is the sole surviving example of an ancient building that retains considerable traces of wall paintings in their original form. Even though they are faded and looking some what forlorn in their present crumbling atmosphere. they will not disappoint you.