How many images contain 'bridge' as an object element?
images refers to IMG_ID; 'bridge' as an object element refers to OBJ_CLASS = 'bridge'
SELECT COUNT(DISTINCT T1.IMG_ID) FROM IMG_OBJ AS T1 INNER JOIN OBJ_CLASSES AS T2 ON T1.OBJ_CLASS_ID = T2.OBJ_CLASS_ID WHERE T2.OBJ_CLASS = 'bridge'