Reliable way to download textures

Is there a reliable way to download textures from http server ? At the moment I tend to get questions marks from time to time and it seems to be random. I am using .png format images. Suggestions welcome, thanks.

private IEnumerator setQuestionImage(string urlk,int index){
	Texture2D tex = new Texture2D(2, 2,TextureFormat.DXT5Crunched, false);
	string url = ""+urlk;
	WWW www = new WWW(url);
	yield return www;
	this.ret = Sprite.Create(tex,new Rect(0, 0, tex.width, tex.height),new Vector2(0.5f, 0.5f));

Check for errors before loading the texture.

if(www.error != null)
     // Load texture

www.Dispose() should be used to abort downloading process. I don’t see any use of the call in your implementatin.