Why don't my sprites animate when I add a self.image in update function?

Question

I'm currrently learning pyglet, and there is a teeny problem. I called a superclass of the init of pyglet.sprite.Sprite in my player class, but when I try to change the self.image from the update function (or any function that is not the init), it doesn't animate anymore, in fact, it just displays the first element in the array conatining the images for my animation. Thanks. Here is are my codes:

import pyglet
from pyglet.window import key
import physicalobject, resources

class Player(physicalobject.PhysicalObject): #PhysicalObject is just another pyglet.sprite.Sprite superclass
    def __init__(self, *args, **kwargs):
        super(Player,self).__init__(img=resources.right_anim, *args, **kwargs)
        # some codes here....
    def update(self, dt):
        super(Player,self).update(dt)
        #... codes
        if self.look_right:
            self.image = resources.right_anim
        elif self.look_left:
            self.image = resources.left_anim