with(obj_name_here)image_speed=0.5;
I think when you set a sprite using D&D there is an option for the subimage number.. Maybe that works like image_single instead of image_index, so you need to use -1 to make it animated.
(o)=instance_create(x,y,obj_name);(o).image_speed=0.5;(o).image_single=-1;