How do I take maximum screenspace for Gui

I have been trying all day to have a Gui Texture 2D pop up and fill the screen. For some reason, using Screen.width and Screen.height is not working. I've stripped the code to the bare minimum (just displaying Gui) (the pixel values for length and width were arbitrarily chosen). If someone could tell me how to have the Texture 2D fill the screen, that would be fantastic.

 function OnGUI () {
 if (canrun == false) {
 return;
 
 } if (ScaryImageChoice1.scaryimage1bool == true) {
 GUI.Label (Rect (0,0,320,480), scaryimage1);
 } 
 
 else if (ScaryImageChoice1.scaryimage2bool == true) {
 GUI.Label (Rect (0,0,320,480), scaryimage2);
 
 } else if (ScaryImageChoice1.scaryimage3bool == true) {
 GUI.Label (Rect (0,0,320,480), scaryimage3);
 }
 }