How many images have objects with the attributes of polka dot?
attributes of polka dot refer to ATT_CLASS = 'polka dot'; images refer to IMG_ID;
SELECT COUNT(T2.OBJ_SAMPLE_ID) FROM ATT_CLASSES AS T1 INNER JOIN IMG_OBJ_ATT AS T2 ON T1.ATT_CLASS_ID = T2.ATT_CLASS_ID WHERE T1.ATT_CLASS = 'polka dot'