How many images include the "wood" objects?
images refer to IMG_ID; "wood" objects refer to OBJ_CLASS = 'wood';
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 = 'wood'