Populate inventory with item icons

Hello everyone. I have my inventory window setup the way I want, but right now it only displays the number of the slot. I've been using a mish mash of tutorials that explain how to populate the inventory with random items from chests etc but I have a set number of items I need to place in the inventory. How can I get the item icons to show up in the slots?

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 
 public class MyGUI : MonoBehaviour {
     public GUISkin mySkin;
 
     public  float lootWindowHeight;
     
     public float buttonWidth = 40;
     public float buttonHeight = 40;
     private float _offset = 10;
     
     /********************************************************/
     /* Inventory Window Variables*/
     /********************************************************/
     private bool _displayInventory = true;
     private const int INVENTORY_WINDOW_ID = 1;
     private Rect _inventoryWindowRect= new Rect(10,10,170,265);
     private int _inventoryRows = 6;
     private int _inventoryCols = 4;
     private const string ARMY_MEN_PATH = "Item Icons/Army Men"; 
     // Use this for initialization
     
     void Start () {
         
     }
     
     // Update is called once per frame
     void Update () {
     
     }
     
     void OnGUI(){
         _inventoryWindowRect = GUI.Window(INVENTORY_WINDOW_ID, _inventoryWindowRect,InventoryWindow, "Inventory");
     }
 
     public void InventoryWindow(int id){
         int cnt = 0;
         for(int y = 0;y < _inventoryRows;y++){
             for(int x = 0;x < _inventoryCols;x++){
                 GUI.Button(new Rect(5 + (x * buttonWidth),20 + (y * buttonHeight),buttonWidth, buttonHeight),(x + y * _inventoryCols).ToString());
                 GUI.Button(new Rect(5 + (x * buttonWidth),20 + (y * buttonHeight),buttonWidth, buttonHeight),(x + y * _inventoryCols).ToString(),"box");    
                 cnt++;
             }
         }
         GUI.DragWindow();    
     }
     
     public void ToggleInventory(){
         
     }
 
 }