How to GUI texture image change?

Dude, changing the texture of a guitexture is a simple process, you can do it either with a GUI Skin as suggested by PushpaK , or check this example.. it might help you.. just comment if anything is not clear about the code

 public var yourGUITexture:GUITexture;
 public var texture1:Texture2D;
 public var texture2:Texture2D;
 private var yourCurrentTexture:Texture2D;
 private var showAlert = false;


 // whenever you will show the window just set showAlert = true;

 function OnGUI()
 {
     if(showAlert)    
        var window = GUI.Window(0 , Rect(posX,posY,someWidth,someHeight), ShowAlert ,"");

     yourGUITexture.texture = yourCurrentTexture;
 }

 function ShowAlert(windowID : int)
 {
      if (GUI.Button (Rect(posX,posY,someWidth,someHeight), "LeftButton")) 
 {      
     showAlert= false;
     yourCurrentTexture = texture1;
 } 

     if (GUI.Button (Rect(posX*2,posY,someWidth,someHeight), "RightButton")) 
 {      
     showAlert= false;
     yourCurrentTexture = texture2;
 } 
   
 }