GUI Error:
You are pushing more GUIClips than you are popping.
Make sure they are balanced (type:Layout)

I think it's pointing to this code: (it is slighty modified - the real code is far more complex ;) )

var imageTexture:Texture=null;
var MyLib:MyLibClass=null;
var p:MyPictureObject=null;

function Start(){
    p=MyLib.GetARandomPicture();
}

function OnGUI(){
    if (imageTexture==null){
        var url=p.GetImage();
        if (url!="" && p.IsBusy()==false){
            p.SetBusy(true);
            Debug.Log("getting an image texture",this);
            if (p.GetImageTexture()!=null){
                Debug.Log("getting an image texture from cache",this);
                imageTexture=p.GetImageTexture();
            } else {
                Debug.Log("getting an image texture from web: "+url,this);
                var www=new WWW(url);
                yield www;
                p.SetimageTexture(www.texture);
                imageTexture=www.texture;
            }   
            p.SetBusy(false);
        }
    }
    GUILayout.Box(Rect(0,0,100,100), imageTexture);
    if (GUILayout.Button("a different image please")){
        p=MyLib.GetARandomPicture();
    }
}

I use p.SetBusy() and p.IsBusy() to avoid multiple requests for one image.

I use p.GetImageTexture() and p.SetImageTexture() for caching request responses.

So If I use caching and avoid asyncronous problems - why do I get something like a stackoverflow on the request buffer?