I can't change a texture from assests but from HTTP

I Dont know what I'm doing wrong I can change the texture from my GameObject "targetItem" to some random picture from the Internet. But when i try to change it to the Texure "wtexture" it just turns "targetItem" white. After i declared "wtexture" a Texture object I selected via the GUI which Texture from the assets should be linked to it none of the Texture from the assets work so I guess they are not correctly linked?

Thanks in advance

 var targetItem : GameObject;
 var wtexture : Texture;
 
 function OnGUI () {
     if (GUI.Button (Rect (10,10,150,50), "World Texture")) {
         print ("You clicked the button!");
         targetItem.renderer.material.mainTexture = wtexture;
     }
     
         if (GUI.Button (Rect (320,10,150,50), "Http Texture")) {
         print ("You clicked the button!");
         httpTextureLoad();
 
     }
 }
 
 
 function httpTextureLoad() {
         
         var url = "http://images.zeit.de/politik/ausland/2013-02/casa-pound/casa-pound-540x304.jpg";
              // Start a download of the given URL
         var www : WWW = new WWW (url);
 
         // Wait for download to complete
         yield www;
     
         // assign texture
         targetItem.renderer.material.mainTexture = www.texture;
 
 }