Почему после второго нажатия на кнопку вылетает ошибка? - C#

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Net;
 
namespace video
{
    public partial class Form1 : Form
    {
        public WebClient a = new WebClient();
        public Form1()
        {
            InitializeComponent();
        }
 
        private void pictureBox1_Click(object sender, EventArgs e)
        {
                a.DownloadFile(@"http://195.5.39.15/cgi-bin/jpg/image.cgi", @"C:\foto.jpg");
                pictureBox1.Image = Image.FromFile(@"C:\foto.jpg");
        }
    }
}