How to download an image and stock it on the device in C# ?

Hi !

I know how to download an image from an URL and display it :

 using UnityEngine;
 using System.Collections;
 using UnityEngine.UI;
 
 public class DownloadImage : MonoBehaviour {
 
     public RawImage img;
 
     IEnumerator Start () {
         WWW www = new WWW("MyURL");
         yield return www;
         img.texture = www.texture;
         img.SetNativeSize();
     }
 
 }

But I don't want to display it immediatly, I want to download all my images, and each time put it in the persistent data on the device and in the same movement rename it to give it an ID. I need some help to stock it, I tried with File.WriteAllBytes but It won't work on a RawImage.

I need some help, thank you for helping me !