Using WWW in public method of a GameObject in Unity C#

I am beginner in Unity3d and C# and after hours of searching solutions (and building a XML reader in C# based on documentation) I stuck on this problem:

I have two prefab planes tagged as "test" and I want to show images from URL on them as texture when the images loaded.

The planes has an empty Start and Update method and this public one in the loadImageFromURL.cs:

 public IEnumerator changeTexture(string url){
         Debug.Log (url);
     
         WWW www = new WWW (url);         
         yield return www;
         renderer.material.mainTexture = www.texture;
 }

Then I have an empty gamobject that collects image URLs in the Start() method then I do this:

 IEnumerator Start () {
 
     WWW www = new WWW("http://example.com/xmlfile.rss");
     yield return www;
     stream =  new StringReader(www.text);
 
     /*Here I do all the XML stuff, it provides the URLs from the rss feed there is no error in it*/
 
     picture =  GameObject.FindGameObjectsWithTag("test");
     loadImageFromURL sc = (loadImageFromURL)picture[0].GetComponent(typeof(loadImageFromURL));
 
     //here I call the function
     sc.changeTexture(URLs[2]);
 }

When I simply pass the URL and just return null; in the changeTexture method it show the url when I Debug.Log it. When WWW lines are there it is like nothing happens. Also this exact same WWW code works perfectly as seen in Unity reference too

So I am pretty sure there is something I do not understand about WWW