How do I progressively download images with WWW.

I am trying to fetch Facebook friend mugshots; which is currently working fine- but now I would like to quickly load low-res images first, then replace them with high-res ones that take longer to download- and that isn't working.

Only the first set comes through. The GetBetterPics function originally did exactly the same thing as the first function- create a new Texture2D, download into it, then assign it to the appropriate icon's material- that didn't work, I changed it to load directly into the material, still doesn't work. What am I doing wrong?

     ...
     suggestedTen.Add(allFriendsSorted[f]);
     var newFace : Texture2D = new Texture2D(32, 32, TextureFormat.DXT1, false);
     var url : String = "https://graph.facebook.com/"+allFriendsSorted[f]+"/picture?width=32&height=32";
     var www = new WWW(url);
     yield www;
     www.LoadImageIntoTexture(newFace);
     friendFaceRenderers[f].material.mainTexture = newFace;
     }
 Invoke ("GetBetterPics",2.0);
 }


  function GetBetterPics ()
     { 
     for (var f : int = 0; f < 10; f++)
         {
         var url : String = "https://graph.facebook.com/"+suggestedTen[f]+"/picture?width=128&height=128";
         var www2 = new WWW(url);
         yield www2;
         www2.LoadImageIntoTexture(friendFaceRenderers[f].material.mainTexture);
         }
     }