List all the ID of the images that have an attribute class of "horse".
ID of all images refer to IMG_ID; attribute class of "horse" refers to ATT_CLASS = 'horse';
SELECT 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 = 'horse'