Question: How many images have at least one pair of object samples with the relation "parked on"?
SQL: SELECT SUM(CASE WHEN T1.PRED_CLASS = 'parked on' THEN 1 ELSE 0 END) FROM PRED_CLASSES AS T1 INNER JOIN IMG_REL AS T2 ON T1.PRED_CLASS_ID = T2.PRED_CLASS_ID WHERE T2.OBJ1_SAMPLE_ID != T2.OBJ2_SAMPLE_ID