using System.Drawing, openFileDialog, DialogResult.OK, Image.fromFile()

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

namespace WindowsFormsApplication5
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            if (openFileDialog1.ShowDialog(this) == DialogResult.OK)
            {
                Image img = Image.FromFile(openFileDialog1.FileName);
                Form1.ActiveForm.BackgroundImage = img;
            }
       
        }
    }
}