Taking screenshots on iOS

I'm using the following code to take pictures of my game, and then using Prime 31's plugin to post these pictures to twitter. The pictures aren't showing up though, and I'm not sure why. I'd appreciate any ideas.

 void Awake()
      {    
          pictureFilePath = Application.persistentDataPath + "/ScreenShot.png";
      }
  
      public void TakePicture()
      {        
          Application.CaptureScreenshot(pictureFilePath);
      }
  
      public void PostToTwitter()
      {
          TakePicture ();
          TwitterBinding.showTweetComposer("Test", pictureFilePath, "https://twitter.com/matski53");
      }
  
      public void PostToFacebook()
      {
          TakePicture ();
          FacebookBinding.showFacebookComposer("Test", pictureFilePath, "https://twitter.com/matski53");
      }