Ok, it turns out that this is a known bug in mayavi. However, it is possible to change the orientation, position and scale of an ImageActor
object after it has been created:
obj = mlab.imshow(img)
obj.actor.orientation = [0, 0, 0] # the required orientation
obj.actor.position = [0, 0, 0] # the required position
obj.actor.scale = [0, 0, 0] # the required scale