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

add

public GUISkin mySkin;

after the OnGui function has to be

void OnGui(){

 GUI.skin = mySkin;
 menuFunction();

}

after some research i have found that i need to call the image from my materials using the following code

private Texture btnStart;

btnStart = (Texture)Resources.Load(“btnStart”);

if (GUI.Button (new Rect((screenWidth - buttonWidth) * 0.7f, screenHeight * 0.15f, buttonWidth * 0.4f, buttonHeight * 0.4f), btnStart))

Thank you for your attempts