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?

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

 function GetBetterPics ()
    for (var f : int = 0; f < 10; f++)
    	var url : String = ""+suggestedTen[f]+"/picture?width=128&height=128";
    	var www2 = new WWW(url);
    	yield www2;

It was because of that stupid “Implicit Downcast” warning I was getting but didn’t know what to do about.
All I had to do to make the warning go away and fix my bug was this:

var tex : Texture = newFace as Texture;
friendFaceRenderers[f].material.mainTexture = tex;