Random picture from class in button (C#)

I have to make a little game for school where you have flags of countries and their names (in a listbox (named lstCountries) for me) and you need to combine them to score points.

I want to get random flags in buttons which I create like this

//Buttons Row 1
            for (int i = 0; i < 5; i++)
            {
                Button btnNew = new Button();
                btnNew.Location = new System.Drawing.Point(230 + 110 * i, 100);
                btnNew.Name = "btnFlag";
                btnNew.Size = new System.Drawing.Size(100, 100);
                btnNew.TabIndex = 0;
                btnNew.Text = "";
                btnNew.UseVisualStyleBackColor = true;
                btnNew.Click += new System.EventHandler(buttonToevoegen);
                Controls.Add(btnNew);
                aButtons.Add(btnNew);
            }

And I made a class named Country:

class Country
    {
        public string Name { get; set; }

        public string Flag { get; set; }

        public int Number { get; set; }
    }

And I created the countries like this:

private void vlaggenDeclareren()
        {
            Country oAlbanie = new Country();
            oAlbanie.Name = "Albanie";
            oAlbanie.Flag = "Albanie.png";
            oAlbanie.Number = 0;
            lstLandVullen(oAlbanie);

            Country oAndorra = new Country();
            oAndorra.Name = "Andorra";
            oAndorra.Flag = "Andorra.png";
            oAndorra.Number = 1;
            lstLandVullen(oAndorra);

            //and so on..
        }

I thought I would be able to give each button a random number from 0-40 (have 40 flags and 20 buttons), and link that random number to countryname.Number

If someone would be able to help me out (even if it's a completely different way) that would be awesome.. We're supposed to do this in a group of 3 people but everyone's refusing to do anything so I'm doing this alone ..