onGui - Switching between textures on selection grid.

Morning guys,

I have made a level select screen for my iphone/ipad game using a selection grid all looks good but I ran into a problem as I want some of the textures to have a locked textures and some to be playable. So when a user completes level 1 the texture for level 2 changes from locked to a new texture.

Just wondering where I would put the if statement? Any help would be appreciated. Thank you!

Here is my code so far -

 var selGridInt : int = 0;
 var selStrings : String[] = ["1", "2", "3", "4", "5", "6", "7", "8", "9", "10", "11", "12", "13", "14", "15", "16", "17", "18"]; 
 public var customSkin : GUISkin;
 public var menuStyle = new GUIStyle();

 function Awake(){
 useGUILayout = false;
 selGridInt = -1;
  }

 function OnGUI () {
 GUI.skin = customSkin;
 selGridInt = GUI.SelectionGrid (Rect (Screen.width / 24, Screen.width / 6, Screen.width / 1.08, Screen.height / 1.6), selGridInt,    selStrings, 6); 
 if(selGridInt > -1){
 Application.LoadLevel(selGridInt);
  }
  }