Error when importing Image per script

Hello! I try to import a picture into my game by using this code:

 IEnumerator LoadTileset(){
         WWW www = new WWW ("C:/Users/Public/Documents/Unity Projects/Your RPG/Assets/Tilesets/Terrain.png");
         yield return www;
         if (www.error == null) {
             source = www.texture;
         } else {
             print ("www Error : " + www.error);
         }
     }

the file I want to import does exist (if I paste the link Into the Windows explorer, it is opening the picture). The File is from a folder created by script at the application folder but when i Start the Coroutine i get this error:

Does someone of you know how to fix this error? Thank you for your help! ^^