C# how to change an image sequentially by clicking on a picturebox

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C#

I have been looking for different solutions and when I finally thought I had it, an "Unhandled Exception" appears. I am a student and I am just starting out and I wanted to see if you could offer me alternatives.

The problem is the following:

"Design a program that determines the amount to be paid and the preview gift for a customer of the shop.

In addition, when the form is executed, it must show a photo of a client, and then when you click
The photo must show a photo of another client up to 3 photos."

What I have tried:

This is the code that I have been doing but I keep getting "unhandled exception"

private void Form1_Load(object sender, EventArgs e)
        {
            picfoto.Image = Image.FromFile("Foto1.jpg");
            a = 1;
        }

 private void picfoto_Click(object sender, EventArgs e)
        {
            switch (a)
            {
                case 1:
                    picfoto.Image = Image.FromFile(Application.StartupPath + "Foto2.png");
                    a = a + 1;
                    break;
                case 2:
                    picfoto.Image = Image.FromFile(Application.StartupPath + "Foto3.png");
                    a = a + 1;
                    break;
                case 3:
                    picfoto.Image = Image.FromFile(Application.StartupPath + "Foto4.png");
                    a = 1;
                    break;

            }