How to resize a normal image then apply as a texture for GUI.Box()?

What I am doing here is actually I have declared a public variable as a Texture2D now I am resizing it to screen width and screen height as I have a gui box of same size.

 var bgImage : Texture2D;
 
 function start(){
 
     bgImage.Resize(Screen.width,Screen.height);
     bgImage.Apply();
 }

Then I am directly using this "bgImage" for guibox which is also of screen.width and screen.height but its not working.

I checked import setting of image and made Read/Write true, and then it is saying format is compressed but when I change it to any other format Image becomes white.