Update a Sprite in Scene

Hey Guys,

i am trying to download some pictures from URL and show them in a Image. What i need to do now is to update the Image. That does not work and i dont know why.

   string url = "https://docs.unity3d.com/uploads/Main/ShadowIntro.png";
   string url1 = "http://images.earthcam.com/ec_metros/ourcams/fridays.jpg";
   Image image;
   List <Sprite> sprite_test = new List<Sprite>();

   IEnumerator Start()
   {
     // Start a download of the given URL
     using (WWW www = new WWW(url))
     {
       // Wait for download to complete
       yield return www;
      // assign texture
       image = GetComponent<Image>();      
       image.sprite = Sprite.Create(www.texture, new Rect(0, 0, www.texture.width, www.texture.height), new Vector2(0, 0));
       image.sprite.name = "Pic1";
       sprite_test.Add(image.sprite);
     }
     using (WWW www = new WWW(url1))
     {
       // Wait for download to complete
       yield return www;
       // assign texture
       image.sprite = Sprite.Create(www.texture, new Rect(0, 0, www.texture.width, www.texture.height), new Vector2(0, 0));
       image.sprite.name = "Pic2";
       sprite_test.Add(image.sprite);
     }
   }
   // Update is called once per frame
   void Update ()
   {
   foreach(Sprite sprite in sprite_test)
     {
       image.sprite = sprite;
     }  
     }

It would be awesome if anybody can help me to figure that out.