Count the image numbers that contain the "paint" object.
image numbers that contain the "paint" object refer to IMG_ID where OBJ_CLASS = 'paint';
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 = 'paint'