I have created a button with photoshop and i need to use it in my menu

I have created a button with photoshop and i am trying to use it instead of my buttons I created.

Till now I have a script with C# that is attached to the main camera. The script basically makes the main menu for me but i would like to change the buttons generated with an image I designed.

This is what I have till now:

using UnityEngine; using System.Collections;

public class myMenu : MonoBehaviour {

 private delegate void MenuDelegate();
 private MenuDelegate menuFunction;
 
 private float screenHeight;
 private float screenWidth;
 private float buttonHeight;
 private float buttonWidth;
 
 // Use this for initialization
 void Start () {
     screenHeight = Screen.height;
     screenWidth = Screen.width;
     
     buttonHeight = screenHeight * 0.3f;
     buttonWidth = screenWidth * 0.4f;
     
     menuFunction = anyKey;
 }
 
 void OnGUI()
 {
     menuFunction();
 }
 
 void anyKey()
 {
     if(Input.anyKey)
     {
         menuFunction = mainMenu;
     }
     
     GUI.skin.label.alignment = TextAnchor.MiddleCenter;
     GUI.Label(new Rect(screenWidth * 0.45f, screenHeight * 0.45f, screenWidth * 0.1f, screenHeight * 0.1f), "Press any key to continue");
 }
 
 void mainMenu()
 {
     // Main Menu
     // Start Game
     // How to Play
     // Settings
     // Quit
             
     
     GUI.Box(new Rect((screenWidth - buttonWidth) * 0.6f, screenHeight * 0.05f, buttonWidth * 0.7f, buttonHeight * 2.5f), "Main Menu");
     
     if (GUI.Button (new Rect((screenWidth - buttonWidth) * 0.7f, screenHeight * 0.15f, buttonWidth * 0.4f, buttonHeight * 0.4f), "Start Game"))
     {
         Application.LoadLevel ("Level1");
     }
     
     if (GUI.Button (new Rect((screenWidth - buttonWidth) * 0.7f, screenHeight * 0.3f, buttonWidth * 0.4f, buttonHeight * 0.4f), "How to Play"))
     {
         
     }
     
     if (GUI.Button (new Rect((screenWidth - buttonWidth) * 0.7f, screenHeight * 0.45f, buttonWidth * 0.4f, buttonHeight * 0.4f), "Settings"))
     {
         
     }
     
     if (GUI.Button (new Rect((screenWidth - buttonWidth) * 0.7f, screenHeight * 0.6f, buttonWidth * 0.4f, buttonHeight * 0.4f), "Quit Game"))
     {
         Application.Quit();
     }
 }
 

}

My goal at the moment is to find a way how to make the script load the image and once clicked it does something like application.quit.

i have never coded in C# before, just Java and .Net, but i can understand C# since they are similar.

and what if i just simply drag and drop it in front of the main camera, making the camera facing 1 position during the main menu. Will i achieve the same results? what happens if the game gets maximized? will the GUI scatter? Sorry but i am still new to unity