Loading image from external directory at runtime

Hi, I'm trying to load a jpg image to then put into file, but I'm getting a NullReference exception at line 25 (tex.loadimage). I can't figure out the reason, can you?

     Sprite map; 
     Texture2D tex;
     WWW www;
     bool canLoad = true;

     void Start () {
         map = gameObject.GetComponent <Sprite> ();
     }
 
     void Update () {
         if (canLoad) {
             if (Input.GetKeyUp (KeyCode.P)) {
 
 
                 StartCoroutine (GitMap ("file://" + Application.dataPath.Remove (20) + "/myMap.jpg"));
 //A mess, but I found no other way to get relative directory to desktop
 
                 Debug.Log (www.url);
                 //This shows that the directory is correct (file myMap on Desktop)
 
                 if (www != null) {
                     Debug.Log (www.bytes);
                 }
 
                 tex.LoadImage (www.bytes); //this is the problem.
 
                 map = Sprite.Create (tex, new Rect (0, 0, tex.width, tex.height), new Vector2 (0.5f, 0.5f));
             } 
         }
     IEnumerator GitMap (string PATH) {
         www = new WWW (PATH);
         yield return www;
     }