help for the background image

Hello everyone I have a problem I can not solve I almost put a background image in a guibox, so that (using special script and running) will resize according to the screen resolution used. I tried to do so:

 public Texture2D Wallpaper;
 
 void Start () 
 {
 Wallpaper = Resources.Load ("Wallpaper/image1") as Texture2D;
 }
 
 void OnGUI()
 {
 GUI.Box(new Rect(0,0,Screen.width,Screen.height),Wallpaper);
 }

but the result is this ...

I tried it with GUILayout.box but the 'image is slightly shifted from the coordinates x=0 and y=0 and largest of the game screen ...

 public Texture2D Wallpaper;
     
 void Start () 
 {
 Wallpaper = Resources.Load ("Wallpaper/image1") as Texture2D;
 }
     
 void OnGUI()
 {
 GUILayout.Box(Wallpaper);
 }

I'm shooting for hours, but I found it on anything that can help me ... how can I do?

PS: sorry for my english