Stack Overflow Exception & Importing Images So They Can Be Scripted

I'm working on a basic 2D menu system utilizing three scenes for each menu. However, when I actually change scenes I get a Stack Overflow Exception error. What am I doing wrong? Also, I'm having trouble importing image assets to use as a backgrounds and icons. How do I do this as well. For the first problem here's the scripting code in C#:

 public class FScript : MonoBehaviour 
 {
     // Variables
     public int sceneIndex = 0;
     public bool escapeCheck = false;
  // Use this for initialization
  void Start () 
     {
         MainMenu();
  }
  // Update is called once per frame
  void Update () 
     {
         if (Input.GetKeyDown(KeyCode.Escape))
             escapeCheck = !escapeCheck;
  }
     void OnGUI()
     {
         if (!escapeCheck)
         {
             if (sceneIndex == 0)
                 MainMenu();
             else if (sceneIndex == 1)
                 GameScene();
             else if (sceneIndex == 2)
                 TestScene();
         }
         else
             showDialog();
     }
     void MainMenu()
     {
         if (GUI.Button(new Rect(150, 150, 100, 100), "START"))
         {
             sceneIndex = 1;
             SceneLoader();
         }
         if (GUI.Button(new Rect(270, 150, 100, 100), "TEST"))
         {
             sceneIndex = 2;
             SceneLoader();
         }
         if (GUI.Button(new Rect(390, 150, 100, 100), "QUIT"))
             Application.Quit();
     }
     void GameScene()
     {
         GUI.Label(new Rect(Screen.width - (Screen.width / 2), Screen.height - (Screen.height / 2), 100, 100), "GAME SCENE");
         if (GUI.Button(new Rect(10, Screen.height - 280, 270, 270), "BACK"))
             SceneLoader();
         OnGUI();
     }
     void TestScene()
     {
         GUI.Label(new Rect(Screen.width - (Screen.width / 2), Screen.height - (Screen.height / 2), 100, 100), "TEST SCENE");
         if (GUI.Button(new Rect(10, Screen.height - 280, 270, 270), "BACK"))
             SceneLoader();
         OnGUI();
     }
     void showDialog()
     {
         GUI.BeginGroup(new Rect(Screen.width - (Screen.width / 2) - 100, Screen.height - (Screen.height / 2) - 100, 300, 300), "Do you want to return to the main menu?");
         if (GUI.Button(new Rect(1, 100, 90, 55), "YES"))
         {
             sceneIndex = 0;
             escapeCheck = false;
             SceneLoader();
             OnGUI();
         }
         if (GUI.Button(new Rect(150, 100, 90, 55), "NO"))
             escapeCheck = false; 
         GUI.EndGroup();
     }
     void SceneLoader()
     {
         if (sceneIndex == 0)
             Application.LoadLevel("MainMenu");
         else if (sceneIndex == 1)
             Application.LoadLevel("GameScene");
         else if (sceneIndex == 2)
             Application.LoadLevel("TestScene");
     }
 }