C# Change button background image using method : My code for FORM

    using System;
    using System.Collections.Generic;
    using System.ComponentModel;
    using System.Data;
    using System.Drawing;
    using System.Linq;
    using System.Text;
    using System.Windows.Forms;

    namespace YAHTZEE
    {
        public partial class GameMainWindow : Form
        {
            public GameMainWindow()
            {
                InitializeComponent();
            }

            private void buttonRollDice_Click(object sender, EventArgs e)
            {
                DiceManager dm = new DiceManager();
                dm.intDice_1_Roll();
                dm.intDice_2_Roll();
                dm.intDice_3_Roll();
                dm.intDice_4_Roll();
                dm.intDice_5_Roll();

                Class1 c1 = new Class1();
                c1.buttonDice_1Manager();

            }
        }
    }

My class for generating random number for all the dice.
This works perfectly fine.

    using System;
    using System.Collections.Generic;
    using System.ComponentModel;
    using System.Data;
    using System.Drawing;
    using System.Linq;
    using System.Text;
    using System.Windows.Forms;

    namespace YAHTZEE
    {
        public class DiceManager
        {
            Random rnd = new Random();

            public int intDice_1_Roll()
            {
                int intDice_1 = rnd.Next(1, 7);
                return intDice_1;
            }

            public int intDice_2_Roll()
            {
                int intDice_2 = rnd.Next(1, 7);
                return intDice_2;
            }

            public int intDice_3_Roll()
            {
                int intDice_3 = rnd.Next(1, 7);
                return intDice_3;
            }

            public int intDice_4_Roll()
            {
                int intDice_4 = rnd.Next(1, 7);
                return intDice_4;
            }

            public int intDice_5_Roll()
            {
                int intDice_5 = rnd.Next(1, 7);
                return intDice_5;
            }
        }
    }

Class to change button background image.
This is where my problem is, the code works if I put it on my FORM but it does nothing when I make it a method on another class.

    using System;
    using System.Collections.Generic;
    using System.Linq;
    using System.Text;
    using YAHTZEE.Properties;

    namespace YAHTZEE
    {
        class Class1 : GameMainWindow
        {
            public void buttonDice_1Manager()
            {
                DiceManager dm = new DiceManager();

                if (dm.intDice_1_Roll() == 1)
                {
                    buttonDice_1.BackgroundImage = Properties.Resources.Dice1;
                }
            }
        }
    }

I'm new to c# but I'm pretty sure this should work. Or am I missing something?
I really need an answer ASAP. Thanks!

P.S. I want them to be separated because there are a lot of things to consider making my code very long.