How many images have "picture" as their attribute class?
images have "picture" as their attribute class refer to IMG_ID where ATT_CLASS = 'picture';
SELECT COUNT(T2.IMG_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 = 'picture'