Unity error while loading image from Windows Phone gallery

I have developped a Windows Phone plugin for my Unity application which enable me to open the Windows Phone gallery in order to select an image and get its physic path.

I've used this path to load the image in the application with a WWW object but I get this error

"Error. Operation has failed with error 0x80070005: access is denied."

The path is correct because I display it.

Here is my test code to load the image with the path.

 IEnumerator loadImage()
 {
     this.guiText.text = filePath;
     WWW www = new WWW(filePath);
     yield return www;

     if(www.error == null)
     {
         GameObject texture = GameObject.Find("UNITY");
         if(texture != null)
         {
             texture.guiTexture.texture = www.texture;
         }
     }
 }

Any help would be appreciated ! Thanks in advance !