設定pictureBox1 圖片自適應：

SizeMode:StretchImage

BackgroundImagelayout : Stretch;

點選開啟檔案效果如下：

程式碼實現：

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

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        public static string path = "d:\\robot\\images";                       //圖片路徑
        private void Form1_Load(object sender, EventArgs e)
        {
            if (!Directory.Exists("d:\\robot\\images"))//若資料夾不存在則新建資料夾   
            {
                Directory.CreateDirectory("d:\\robot\\images"); //新建資料夾   
            }
            pictureBox1.BackgroundImage = Image.FromFile(path + "\\sh.png");
        }

        private void btn_Click(object sender, EventArgs e)
        {
            System.Diagnostics.Process.Start(path);
        }
    }
}